Docs: build directly using sphinx

[tobiasdiez]

Instead of our custom docbuilder, build the documentation simply by running:

python -m sphinx ./builddir/src/doc/en/ ./builddir/src/doc/html/en/

(has to be run after an initial meson compile -C builddir doc-html which generates some of the rst files)

The old way to build the docs is not (yet) removed and can be activated via:

meson configure builddir -Dlegacy-docs=true
meson compile -C builddir doc-html

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies