refactor: removes files now provided in canonical-sphinx==0.3.0

[SecondSkoll]

• Removes superfluous files, adding a custom CSS placeholder
• Changes source of required package
• moves gitignore file to root, and changes referenced files and directories to support nested paths

[tang-mm]

The changes look good to me, but apparently some Sphinx internal dependencies (pillow/ wheel) caused the build failure. Could you verify whether any upstream change is involved?

[SecondSkoll]

The changes look good to me, but apparently some Sphinx internal dependencies (pillow/ wheel) caused the build failure. Could you verify whether any upstream change is involved?

Looks like it might need an updated version of Python to build...
Pillow for Python 3.12 builds with Python 3.12.8, and the default Ubuntu latest image used as the runner base uses 3.12.3. I'm not entirely sure though, so I'll have to have a tinker.

Edit: This was not correct, but was easy to check at least.

[SecondSkoll]

It appears to be an issue with 24.04 runners not having some of the prerequisites for building Pillow from source. I've added the required packages.

[tang-mm]

LGTM 🚀