Docs refresh: README, CONTRIBUTING, and misc updates

[ScriptAutomate]

What does this PR do?

README preview: https://github.com/ScriptAutomate/salt/blob/refresh-docs/README.rst

A handful of things:

• Update all python.org/2/* URI links in doc/*.rst files (many links still need to be updated in *.py files in the Salt repo)
• Move contents of doc/topics/development/contributing.rst to new CONTRIBUTING.rst file at root, and import it at build with Sphinx. CONTRIBUTING.rst now new, primary contributing guide.
• Delete redundant contributor guide files: CONTRIBUTING.md and .github/CONTRIBUTING.md
• Refresh README.rst
• Include README.rst contents in doc/contents.rst
  • Updated doc/_themes/saltstack2/layout.html so TOC link still works as expected
• Delete doc/.scripts/setup-transifex-config: No longer used
• Correct Aluminum to Aluminium
• Replace some straggling documentation about develop branch that need to be referencing master branch for development. Also mentioning tag releases instead of branch releases.
  • See SEP 14

What issues does this PR fix or reference?

• Fixes: [DOCS] Contributor Guide: Needed Review, Refresh, and Jumping Off Point #58703
• Fixes: [DOCS] Adding raw directive to README.rst fails twine check #58727 (as this PR removes the .. raw:: directive)
• Fixes [BUG] A couple of README badges are broken #59365 by removing latest release badge and updates Twitter badge
• Fixes: [BUG] #52359 was fixed for develop, but not merged to master #58313 (in regard to updating docs to consistently approach development against the master branch)
  • That ticket also asks about information regarding how the PR jams work, and what the process is for migrating PRs from the develop branch into the master branch. This PR is not resolving that, and I don't think the official docs should have this information, but should exist as an ongoing effort detailed elsewhere due to the dynamic nature of PR jams (and how the develop branch will eventually be deleted after all appropriate migrations to master occur).
• Related to [BUG] Installing Salt for development docs out of date #58516 when it comes to completing some cleanup around contributing docs.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs

Commits signed with GPG?

Yes

[sagetherage]

few changes to the branching here, see my comments

[sagetherage]

Putting a general comment to review the Deprecation docs and Release docs.

[sagetherage]

I am good to merge this in and then re-word the deprecation portion in a separate PR.

[s0undt3ch]

Failures are unrelated

[ScriptAutomate]

Before this is merged, I'd like to just get confirmation that this makes sense:

• Include README.rst contents in doc/contents.rst
  • Updated doc/_themes/saltstack2/layout.html so TOC link still works as expected

This makes contents.rst not only be a TOC, but have the README.rst contents injected directly above the TOC, since the contents page is pretty much the landing page for the salt repo docs. The TOC / Table of Contents buttons and links will still go directly to the section of the TOC for people wanting to look at a main TOC.

I also have updated the Twitter social media button link.

[s0undt3ch]

To me, it makes sense.

[ScriptAutomate]

I've also closed #59383 which is duplicating one of the included fixes in this PR

[ScriptAutomate]

Before this is merged, I'd like to just get confirmation that this makes sense:

* Include `README.rst` contents in `doc/contents.rst`
  
  * Updated `doc/_themes/saltstack2/layout.html` so TOC link still works as expected

This makes contents.rst not only be a TOC, but have the README.rst contents injected directly above the TOC, since the contents page is pretty much the landing page for the salt repo docs. The TOC / Table of Contents buttons and links will still go directly to the section of the TOC for people wanting to look at a main TOC.

I also have updated the Twitter social media button link.

This ended up introducing some unexpected, downstream issues that will be resolved with the following:

• Move README contents; fix TOC for pdfs/epubs; revert saltstack2 TOC modification #59446
• use sphinx.ext.imgconverter #59445
• GitLab / builddocs: https://gitlab.com/saltstack/open/docs/builddocs/-/merge_requests/15