[PR #6045/13c26be8 backport][3.8] Add custom RST roles extlinks to the docs setup

[patchback]

This is a backport of PR #6045 as merged into master (13c26be).

What do these changes do?

This patch declares the following roles within Sphinx:

• issue
• pr
• commit
• gh
• user

They all correspond to respective GitHub URLs. For example,
the following will link a GitHub user page:

    :user:`webknjaz`

(this is useful for crediting people's contributions in the changelog, a trick I've stolen from tox and improved by optionally linking the Sponsors page that is now used by many other projects I maintain and contribute to)

Are there changes in behavior for the user?

Not for the end-users, no.
When the contributors submit changelog fragments, they may optionally add their signatures with this. Or link the PRs and other stuff on GitHub.

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."