meson: build docs with meson

[jelly]

Introduce a custom target build-docs to build the sphinx documentation.

[jelly]

This needs some discussion, do you want a custom target or always build documentation when -Ddocs=true is provided. Either way this seems doable with meson :)

[jelly]

Also FYI, make docs is completely broken now on main:

[jelle@toolbx python-systemd]$ make doc
python -m pip install .
Defaulting to user installation because normal site-packages is not writeable
Processing /home/jelle/projects/python-systemd
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Preparing metadata (pyproject.toml) ... done
Building wheels for collected packages: systemd-python
  Building wheel for systemd-python (pyproject.toml) ... done
  Created wheel for systemd-python: filename=systemd_python-236-cp314-cp314-linux_x86_64.whl size=63848 sha256=042914319b6d0c6d37a15c31ae874ffb62f1c1ddcc2a2942408414a5d052ecca
  Stored in directory: /home/jelle/.cache/pip/wheels/b1/7f/f3/6282dd246c3a670ea95ce507e7d60676a0fbcc7c59e332c369
Successfully built systemd-python
Installing collected packages: systemd-python
  Attempting uninstall: systemd-python
    Found existing installation: systemd-python 236
    Uninstalling systemd-python-236:
      Successfully uninstalled systemd-python-236
Successfully installed systemd-python-236
mkdir build && cd build && python -m sphinx -b html -D version=236 -D release=236 ../docs html
mkdir: cannot create directory ‘build’: File exists
make: *** [Makefile:48: sphinx-html] Error 1