doc: add UV setup details to README.md

[ulysses4ever]

This is towards #11818 @philderbeast @geekosaur Since there was no specification for what the new piece of documentation should achieve, I unleashed my AI bot on it. The result looks legit but maybe you guys had something different in mind?

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Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

[ulysses4ever]

@hasufell can you help with the release pipeline: I don't understand why it runs on PRs like this one --- I thought you said it needs a special label? Also, why it fails here... (see GitHub Actions pane above or here's a direct link to one of the many failing jobs: https://github.com/haskell/cabal/actions/runs/26291120700/job/77391368065?pr=11823)

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-05-24 15:23 UTC · Rule: squash-merge
• ✅ Checks started · on draft merge queue: embarking master (6d67235), #11862 and #11823 together #11877
• ❌ Checks failed
• 🚫 Left the queue — 2026-05-24 17:19 UTC · at b01fa80e900a77ab8264f152e4d588ea141df42c

This pull request spent 1 hour 56 minutes 4 seconds in the queue, including 1 hour 23 minutes 57 seconds running CI.

Waiting for any of

• check-neutral = Validate post job
• check-skipped = Validate post job
• check-success = Validate post job

All conditions

• any of [🛡 GitHub branch protection]:
  • check-neutral = Validate post job
  • check-skipped = Validate post job
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• #approved-reviews-by >= 2 [🛡 GitHub branch protection]
  • doc: add UV setup details to README.md #11823
• #changes-requested-reviews-by = 0 [🛡 GitHub branch protection]
  • doc: add UV setup details to README.md #11823
• #review-threads-unresolved = 0 [🛡 GitHub branch protection]
  • doc: add UV setup details to README.md #11823
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Reason

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