Replace pip with uv for building users guide

[philderbeast]

Fixes #11381. In draft, an experiment.

---

• 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]

I will unsubsccribe from this for now but feel free to explicitly ping me when you need input. I encourage to first discuss the objective of this in ##11381 though.

[philderbeast]

Build times

Comparing the times for ways of building the docs.

• With uv

$ uv cache clean
Clearing cache at: /home/philderbeast/.cache/uv
Removed 3883 files (72.9MiB)
   
$ time make users-guide --always-make
...
________________________________________________________
Executed in    2.05 secs    fish           external
   usr time    1.99 secs  568.00 micros    1.99 secs
   sys time    0.07 secs   89.00 micros    0.07 secs

• Currently without a fix for python-3.13

$ time make users-guide --always-make
make -C doc users-guide
make[1]: Entering directory '/home/philderbeast/dev/src/cabalism/cabal/doc'
python3 -m venv ../.python-sphinx-virtualenv
(. ../.python-sphinx-virtualenv/bin/activate)
mkdir -p ../dist-newstyle/doc/users-guide
(. ../.python-sphinx-virtualenv/bin/activate && pip install -r requirements.txt && sphinx-build -n -W --keep-going -E . ../dist-newstyle/doc/users-guide)
ERROR: Could not open requirements file: [Errno 2] No such file or directory: 'requirements.txt'
make[1]: *** [Makefile:40: ../dist-newstyle/doc/users-guide/index.html] Error 1
make[1]: Leaving directory '/home/philderbeast/dev/src/cabalism/cabal/doc'
make: *** [Makefile:287: users-guide] Error 2

________________________________________________________
Executed in    1.33 secs    fish           external
   usr time    1.25 secs   42.11 millis    1.21 secs
   sys time    0.12 secs   12.12 millis    0.11 secs

• Currently with the fix for python-3.13 (from Bump sphinx and rtd theme #11379)

$ time make users-guide-requirements
...
________________________________________________________
Executed in    2.96 secs    fish           external
   usr time    1.94 secs  912.00 micros    1.94 secs
   sys time    0.23 secs    0.00 micros    0.23 secs

$ time make users-guide --always-make
...
________________________________________________________
Executed in    4.48 secs    fish           external
   usr time    3.91 secs  612.00 micros    3.91 secs
   sys time    0.24 secs   96.00 micros    0.24 secs

[ulysses4ever]

Rebasing on master should solve the ci issue