docs: improve external contribution experience

[weikanglim]

I'm creating this issue to discuss ideas and track improvements around making azd external contributions easier. I've looked into the current state of azd from an external lens, and broadly speaking, I think we want to do something along the lines of the following:

• Rewrite CONTRIBUTING.md to assume limited knowledge with azd for building, running or testing azd
• Move docs and CONTRIBUTING.md to a more visible location. We currently aren't following the best practices for github contributing guides. It should either be under .github or docs. Personally, I've found the cli/azd/docs location to be quite inaccessible. -- For now, we'll keep it as-is since it's linked in the root contributing.md
• Some broad guidelines around pull requests and issues in CONTRIBUTING.md.

[weikanglim]

Was able to find enough time with #4355 to complete the two tasks above, and removed requirement 2 after brief discussion with Matt.