Add properties page, move cell_gate and cell_word under a singular cell_index along with properties. Fix links accordingly.
Also drop x-aware and x-output todos since they are resolved.
Most of the word/coarse level cells have an assigned group and individual page.
The gate/fine level cells are all on one page.
Fix links to `cell_library.rst`.
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.
