Docs: Preliminary autocellgroup usage

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.

docs/source/cell/gate_other.rst

@@ -0,0 +1,5 @@
+.. autocellgroup:: gate_other
+   :caption: Other gate-level cells
+   :members:
+   :source:
+   :linenos:

docs/source/cell/word_other.rst

@@ -0,0 +1,5 @@
+.. autocellgroup:: word_other
+   :caption: Other word-level cells
+   :members:
+   :source:
+   :linenos:

docs/source/cell/word_unary.rst

@@ -0,0 +1,50 @@
+.. role:: verilog(code)
+   :language: Verilog
+
+Unary operators
+---------------
+
+All unary RTL cells have one input port ``A`` and one output port ``Y``. They
+also have the following parameters:
+
+``A_SIGNED``
+   Set to a non-zero value if the input ``A`` is signed and therefore should be
+   sign-extended when needed.
+
+``A_WIDTH``
+   The width of the input port ``A``.
+
+``Y_WIDTH``
+   The width of the output port ``Y``.
+
+.. table:: Cell types for unary operators with their corresponding Verilog expressions.
+
+   ================== ==============
+   Verilog            Cell Type
+   ================== ==============
+   :verilog:`Y =  ~A` `$not`
+   :verilog:`Y =  +A` `$pos`
+   :verilog:`Y =  -A` `$neg`
+   :verilog:`Y =  &A` `$reduce_and`
+   :verilog:`Y =  |A` `$reduce_or`
+   :verilog:`Y =  ^A` `$reduce_xor`
+   :verilog:`Y = ~^A` `$reduce_xnor`
+   :verilog:`Y =  |A` `$reduce_bool`
+   :verilog:`Y =  !A` `$logic_not`
+   ================== ==============
+
+For the unary cells that output a logical value (`$reduce_and`, `$reduce_or`,
+`$reduce_xor`, `$reduce_xnor`, `$reduce_bool`, `$logic_not`), when the
+``Y_WIDTH`` parameter is greater than 1, the output is zero-extended, and only
+the least significant bit varies.
+
+Note that `$reduce_or` and `$reduce_bool` actually represent the same logic
+function. But the HDL frontends generate them in different situations. A
+`$reduce_or` cell is generated when the prefix ``|`` operator is being used. A
+`$reduce_bool` cell is generated when a bit vector is used as a condition in an
+``if``-statement or ``?:``-expression.
+
+.. autocellgroup:: unary
+   :members:
+   :source:
+   :linenos:

docs/source/cell_gate.rst

@@ -3,7 +3,6 @@ Gate-level cells
 
 .. toctree::
    :caption: Gate-level cells
-   :maxdepth: 1
-   :glob:
+   :maxdepth: 2
 
-   /cell/gate_*
+   /cell/gate_other

docs/source/cell_word.rst

@@ -2,8 +2,8 @@ Word-level cells
 ----------------
 
 .. toctree::
-   :caption: Word-level cells
-   :maxdepth: 1
+   :maxdepth: 2
    :glob:
 
-   /cell/word_*
+   /cell/word_unary
+   /cell/word_other