- change documentation theme from `alabaster` to `shibuya`
- add pinned Documentation requirements for `sphinx` and `shibuya`
- update workflows to create `.venv` and install with `requirements.txt`
- ignore common virtual environment directories (`.conda` and `.venv`)
Embed the Icarus Verilog documentation, and format it so that the
Sphinx tools can process it into html or other formats. This will
make the documentation easier to keep up to date with the actual
software.