Add ruff, adjust pyugprade, autoflake in docs

[huonw]

Fixes #19273 by adding ruff to the docs, both the auto-generated backend docs and the manual pages that list out backends. This doesn't try to solve the general problem of reminding us to add backends to docs.

While I was there, I noticed the pyupgrade and autoflake backends were referring to the experimental paths, but they've now graduated to non-experimental, so this switches them over.

[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.

❌ 2.16.x

I was unable to cherry-pick this PR to 2.16.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 2.16.x \
     && git checkout -b cherry-pick-19555-to-2.16.x FETCH_HEAD \
     && git cherry-pick 9b1b322701347a7e260e25e90a859360e1750a5d

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "19555" "2.16.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.

❌ 2.17.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-19555-to-2.17.x FETCH_HEAD \
     && git cherry-pick 9b1b322701347a7e260e25e90a859360e1750a5d

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "19555" "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.

---

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