Add issue and PR templates.

Add a CONTRIBUTING.md and a section in the docs on contributing.
Rm CONTRIBUTORS.md because it is outdated, redundant with Github stats
and hard to maintain with the large amount of new contributors.

Kept the PR template simple to not demotivate people from contributing,
it could be improved with specialized templates in the future if
necessary.
In order to improve bug reports, we offer several templates for
filing issues/bugs.

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve SLEEF
+title: ''
+labels: ''
+---
+
+**Describe the bug**
+A clear and concise description of what the bug is.
+
+**Command lines and logs**
+ - CMake configuration AND build command
+ - CMake configuration logs
+ - CMake build logs (only the relevant part)
+
+**To Reproduce**
+Always try to provide a reproducer, if applicable.
+ - Link to a [Compiler Explorer](https://godbolt.org/) reproducer, if applicable.
+ - Provide source code for reproducer along with command used to build reproducer.
+
+**Expected behavior**
+A clear and concise description of what you expected to happen.
+
+**Screenshots**
+If applicable, add screenshots to help explain your problem.
+
+**Environment**
+ - Is it a native build or cross-compilation?
+ - Host/Target Architecture [e.g. x86_64 if native build, x86_64/aarch64 if cross build, ..]
+ - OS with version [e.g. Linux/Ubuntu-22.04, Windows, macOS, iOS, ...]
+ - CMake version [e.g. 3.18 , ...]
+ - Makefile Generator [e.g. GNU Make, Ninja, ...]
+ - Compiler with version [e.g. gcc-14, clang-18, apple-clang-15.2, msvc-15.0, ...]
+ - If CUDA, please provide the version of the CUDA ToolKit.
+
+**Additional context**
+Add any other context about the problem here.
+

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: 🗣 Ask a Question, Discuss
+    url: https://github.com/shibatch/sleef/discussions
+    about: Get help using SLEEF

.github/ISSUE_TEMPLATE/docs_request.md

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+---
+name: Request Documentation
+about: Request a change or report an issue in documentation
+title: ''
+labels: ["doc"]
+---
+
+**Where?**
+Where in the documentation did you notice an issue, e.g. README.md, `docs/*.md`, `sleef.org/*`, ...
+
+**What?**
+What issue do you want to report? What change do you want to make?
+
+**How?**
+A clear and concise description of what you want to happen.
+
+**Additional context**
+Add any other context or screenshots about the issue here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest a new feature or an idea for SLEEF
+title: ''
+labels: ''
+---
+
+**Why a new feature? Is your feature request related to a problem? Please describe.**
+A clear and concise description of what the problem is, e.g.,
+- I'm always frustrated when [...]
+- [This program] that uses SLEEF fails [in some way], because it is failing to provide [...]
+
+**What feature would you like?**
+A clear and concise description of what feature you want to introduce.
+Is it related to: algorithms, architecture specific code, build system, performance, accuracy, ...
+
+**How would you like this feature to be implemented?**
+A clear and concise description of how you want the feature implemented.
+
+**How portable is the feature across architectures and platforms?**
+A clear and concise description of potential portability issues.
+
+**Describe alternative approaches you've considered**
+A clear and concise description of any alternative approaches you've considered.
+
+**Additional context**
+Add any other context or screenshots about the feature request here.

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for contributing! -->
+
+# Checklist
+
+- [ ] I have read the [contributing guidelines](https://github.com/shibatch/sleef/blob/HEAD/CONTRIBUTING.md).
+- [ ] In particular, I have considered portability of my change across platforms and architectures.
+- [ ] I have self-reviewed my code.
+- [ ] I have commented my code where necessary.
+- [ ] I have updated the documentation accordingly.
+- [ ] I have added tests that prove my fix is effective or that my feature works.
+
+# What is the purpose of this pull request?
+
+<!-- Keep the line(s) that apply. -->
+
+* Fix a bug
+* Improve code quality or performance
+* Add support for a new vector extension
+* Add a workflow to GitHub Actions or add/modify a job in an existing workflow
+* Documentation update
+* Other, please explain:
+
+# What changes did you make?
+
+<!-- Give an overview of the change. -->
+
+# Does this PR relate to any existing issue?
+
+Relates to/Fixes # (issue)
+
+# Is there anything you would like reviewers to focus on?
+
+Please on focus on...

CONTRIBUTING.md

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+# Contributing
+
+Please be sure to read the contribution guidelines before making or requesting a change.
+
+## Design Principles
+
+Before anything please consider what are the main design principle of SLEEF:
+
+1. Portability across platforms and architectures
+2. Compliance to C99 standard
+3. Best performance of vector implementation of branchless and lookup-free algorithms
+
+## Filing Issues
+
+Before filing an issue, please be sure to read the guidelines for what you are reporting:
+
+* [Report Bugs](https://sleef.org/contribute#report-bugs)
+* [Request Feature](https://sleef.org/contribute#request-feature)
+* [Request a Change in Documentation](https://sleef.org/contribute#request-documentation)
+
+## Contributing Code or Documentation
+
+Before submitting code or documentation to SLEEF, please read over the [Pull Request Guidelines](https://sleef.org/contribute#pull-requests).
+
+## Full Documentation
+
+Our full contribution guidelines can be found at: <https://sleef.org/contribute>

docs/6-contribute/README.md

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+---
+layout: default
+title: Contribute
+nav_order: 7
+permalink: /6-contribute/
+---
+
+# Contribute to SLEEF
+{:toc}
+
+## Discussions
+
+If you have a question that does not necessarily require a change to SLEEF,
+such as asking how something works or how to contribute, please [open a
+discussion](https://github.com/shibatch/sleef/discussions/new/choose) instead.
+
+## Templates
+
+By using the provided issues or pull-request template you make sure we have all
+the information we need to triage, understand and solve your issue as quickly
+and efficiently as we can.  Please help us by providing as much information as
+you can by filing the provided forms.
+
+## Report Bug
+
+If you would like to report a bug in SLEEF, please [create a new
+issue](https://github.com/shibatch/sleef/issues/new) and select [Report
+Bugs](https://github.com/shibatch/sleef/issues/new?template=bug_report.md).
+You can also suggest a fix by [submitting a pull
+request](https://github.com/shibatch/sleef/pulls) on GitHub.
+
+## Request Feature
+
+If you would like to request a feature or a change to SLEEF, please [create a
+new issue](https://github.com/shibatch/sleef/issues/new) and select [Request
+Features](https://github.com/shibatch/sleef/issues/new?template=feature_request.md).
+
+## Request Documention
+
+If you would like to report an issue or request a change to the documentation,
+please [create a new issue](https://github.com/shibatch/sleef/issues/new) and
+select [Request
+Docs](https://github.com/shibatch/sleef/issues/new?template=docs_request.md).
+
+## Pull Requests
+
+If you would like to suggest a fix or any change to SLEEF, please [submit a
+pull request](https://github.com/shibatch/sleef/pulls) on GitHub.