Add `doc_string` field to `Pass` constructor
Add `docs/util/newcmdref.py` to contain command domain
Update `docs/util/cmdref.py` with `cmd:usage` and `cmd:optiongroup` for describing commands.
Functional, but WIP.
Properties are both an option:
```
.. cell:def:: $add
:properties: is_evaluable
```
and a field:
```
.. cell:def:: $eqx
:property x-aware:
:property is_evaluable:
```
Properties as an option appear in the property index: linking a given property to all cells with that property; while properties as a field display with the cell.
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.
