Remove custom documentation

[jorgensd]

The current documentation doesn't build on the github ubuntu-latest servers:
https://github.com/FEniCS/basix/actions/runs/18881784871/job/53886616092

This is another reason to go for a standardized sphinx documentation.
The proposed way to go is to use pydata-sphinx-theme, as used in the latest ufl documentation:
https://docs.fenicsproject.org/ufl/main/