Allows for more expressive code when constructing help messages for cells.
Will also move extra logic in parsing help strings into the initial python parse instead of doing it in the C++ at export time.

Include Source file and line number in SimHelper struct, and use it for verilog code caption in rst dump.
Also reformat python string conversion to iterate over a list of fields instead of repeating code for each.

Since `simcells.v` uses consistent formatting we can handle it specifically to help tidy up sphinx warnings about the truth tables, and instead chuck them in a code block which when printing to rst.
Also has the side effect that rst code blocks can be added manually with `//- ::` followed by a blank line.

Generate in a temp directory and use `rsync -rc` to only update rst files that have changed.  This prevents sphinx from having to re-generate every cmd/cell page any time the git sha changes.
Also change cmd gen to match.

Fix `$macc` page.

Subclass the command reference code in order to support smart references to the internal cells.

Use new `autoref` role when using single backticks. Allows automatic mapping to a cmd ref or a cell ref.

- Drop `cell_code` and instead map code lookups to the `cell_help` dict.
- Add helper functions to struct for checking and getting the right cell.
- Add `CellType` for cell to `write_cell_rst` function declaration in
  preparation for use in future.
- Iterate over `yosys_celltypes.cell_types` when exporting cell rst files,
  reporting errors for any cells defined in `cell_types` but not
  `cell_help_messages`.

Use autodocs to perform cell reference docs generation instead of generating rst files directly.
e.g.
```
.. autocell:: simlib.v:$alu
   :source:
   :linenos:
```

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')`.

New `help -dump-cells-json <file>` to dump cells list.
Add 'group' field to SimHelper class/struct with defaults to gate_other and word_other depending on source (simcells or simlib).
Add 'unary' group to unary operator cells for testing (based on internal cell library docs page).

Adds an Appendix header to sidebar toc instead of being an (empty) subpage.

e.g.
```
.. autocell:: $alu
   :source:
   :linenos:
```

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.

Add binary group tag to relevant cells.
Remove content from `cell_library.rst` that is already moved.

Drop the default help message from rst while still displaying it on the command line.
Fix command line formatting for older style help messages.

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`.

`$mux-tree` should be `$mux`-tree.

Move todos to correct place.
Add todo for x-prop cells.

Drop word_other block since it raises a warning, which will cause the RTDs build to fail.

Explicitly assign $_TBUF_ to `gate_other` and remove catch if a cell has no group.

Tags are added to the list of properties when exporting to `cells.json`.

Also tag as x-aware cells and add titles.

These two files can now be safely .gitignore'd.

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.

Also a few extra cell help texts.

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.

Also put property lists *after* cell description.

- Unswap shift/shiftx
- Add brief overview to cell lib
- Clarify $div cell B input
- Clarify unary operators
- What is $modfloor

Also making ver2 cell descriptions consistently spaced.

$lut and $sop were missed in the rebase, and $buf is new to main since the last rebase.

It somehow got lost in the rebase.