update pants peek docs with latest outputs

[tdyas]

Fix #20885 by updating the documentation for pants peek to reflect the current structure of the JSON output.

Closes #20885

[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.

❌ 2.24.x

I was unable to cherry-pick this PR to 2.24.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 2.24.x \
     && git checkout -b cherry-pick-22051-to-2.24.x FETCH_HEAD \
     && git cherry-pick 0ac89eed7a018ee0bcb7dac621d1dc3719eec06f

3. Fix the merge conflicts and commit the changes
4. Run build-support/cherry_pick/make_pr.sh "22051" "2.24.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.

✔️ 2.25.x

Successfully opened #22053.

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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!

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