Meta: make `pyproject.toml` more useful when using dependency managers

**Your Godot version:** 4.6

**Issue description:**

Currently, the tutorial for building the manual suggests using the `requirements.txt`. This misses a few things (such as dependency resolution, although we don't really need that) but mainly it means that no python version is actually being specified by the docs. I believe the versions specified in the `requirements.txt` need python 3.10 or above (at least that's what poetry told me).

This can easily be fixed by adding `[project]` metadata.

This has the added benefit of allowing tools such as poetry and uv to be used when creating venv's to build the docs.

**URL to the documentation page (if already existing):**

https://contributing.godotengine.org/en/latest/documentation/manual/building_the_manual.html