Remove `/source/cell` from .gitignore.
Add a few initial cell pages.
Add YosysCellGroup documenter and cell:group directive.
Update Documenters to use nested json.
Better nested tocs for group.module.source layout.
Fix RTD not including source with warning
`WARNING: invalid signature for autocellsource ('/home/docs/checkouts/readthedocs.org/user_builds/yosys/checkouts/manual-rewrite/source/generated/simlib.v:$alu::__source')`.