[llvm][docs] Minor fixes and improvements for release process

[c-rhodes]

• The list numbering in [1] currently starts again after item 3 due to the code-block.
• Remove mentions of Phabricator and Subversion.
• In final step of [2] remove mention of llvm/utils/git/sync-release-repo.sh, which was removed in [workflows] Remove sync-release-repo.yml #73682.
• Add direct links to:
  • www-releases repo.
  • backporting doc [3].
  • Getting Started page.
  • RELEASE_TESTERS.txt.
  • Release Sources GitHub workflow.

[1] https://llvm.org/docs/HowToReleaseLLVM.html#create-release-branch
[2] https://llvm.org/docs/HowToReleaseLLVM.html#triaging-bug-reports-for-releases
[3] https://llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches

[c-rhodes]

Just a few things I noticed going through the doc Tobias pointed me at. I also had a couple of other questions/observations.

1. I noticed http://prereleases-origin.llvm.org is updated a bit inconsistently and hasn't been updated since Feb 24, is this step still necessary?

   llvm-project/llvm/docs/HowToReleaseLLVM.rst

   Line 407 in 1406058

   access must login to prereleases-origin.llvm.org and manually pull the new

2. llvm-project/llvm/docs/HowToReleaseLLVM.rst

   Lines 362 to 363 in 1406058

   	branch, updating documentation that refers to the release, and updating the
   	demo page.

   - what's the demo page? Also I think it'd be helpful to expand on what docs need updating.

Happy to fix these if you can give me some pointers.