ci: Use uv for install on ReadTheDocs

[matthewfeickert]

Description

RTD doesn't support uv at the same level as pip, but there are ways to still use it along with most of the infrastructure. Adopt the strategy provided in ReadTheDocs's 'Build process customization' section https://docs.readthedocs.io/en/latest/build-customization.html#install-dependencies-with-uv, from readthedocs/readthedocs.org#11152, but continue to monitor alternative faster methods such as those described in readthedocs/readthedocs.org#11289 .

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* RTD doesn't support uv at the same level as pip, but there are ways
  to still use it along with most of the infrastructure. Adopt the
  strategy provided in ReadTheDocs's 'Build process customization' section
  https://docs.readthedocs.io/en/latest/build-customization.html#install-dependencies-with-uv
  , from https://github.com/readthedocs/readthedocs.org/pull/11152/,
  but continue to monitor alternative faster methods such as those
  described in https://github.com/readthedocs/readthedocs.org/issues/11289 .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (58c72b6) to head (07eef5c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2475   +/-   ##
=======================================
  Coverage   98.21%   98.21%           
=======================================
  Files          69       69           
  Lines        4543     4543           
  Branches      804      804           
=======================================
  Hits         4462     4462           
  Misses         48       48           
  Partials       33       33           

Flag	Coverage Δ
contrib	97.79% <ø> (ø)
doctest	98.08% <ø> (ø)
unittests-3.10	96.23% <ø> (ø)
unittests-3.11	96.23% <ø> (ø)
unittests-3.12	96.23% <ø> (ø)
unittests-3.8	96.25% <ø> (ø)
unittests-3.9	96.27% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[matthewfeickert]

This PR drops the install time of pyhf and its dependencies (so '.[docs]') from 52 seconds to 5 seconds.

[matthewfeickert]

I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.