Remove references to PyPI/pantsbuild.pants wheels in docs

[huonw]

This updates our docs to refer to the new release process, using GitHub Releases, instead of PyPI. The pantsbuild.pants package becomes somewhat of an implementation detail too, so I've adjusted these too.

Also, the docs previously referred to the universal PEX we published to GitHub Releases which we no longer do (#19450), and thus references to it are removed too.

[WorkerPants]

I tried to automatically cherry-pick this change back to each relevant milestone, so that it is available in those older releases of Pants.

❌ 2.17.x

I was unable to cherry-pick this PR to 2.17.x, likely due to merge-conflicts.

Steps to Cherry-Pick locally

To resolve:

1. (Ensure your git working directory is clean)
2. Run the following script to reproduce the merge-conflicts:

   git fetch https://github.com/pantsbuild/pants main \
     && git fetch https://github.com/pantsbuild/pants 2.17.x \
     && git checkout -b cherry-pick-19924-to-2.17.x FETCH_HEAD \
     && git cherry-pick 485b2bc7191057b92f7466d3aff544b7f8496588

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "19924" "2.17.x"

Please note that I cannot re-run CI if a job fails. Please work with your PR approver(s) to re-run CI if necessary.

✔️ 2.18.x

Successfully opened #19930.

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When you're done manually cherry-picking, please remove the needs-cherrypick label on this PR.

Thanks again for your contributions!

🤖 Beep Boop here's my run link