Docs: Update for properties

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.

docs/source/cell/properties.rst

@@ -0,0 +1,18 @@
+Cell properties
+---------------
+
+.. TODO:: Fill :cell:ref:`is_evaluable`
+
+.. cell:defprop:: is_evaluable
+
+.. cell:defprop:: x-aware
+
+   Some passes will treat these cells as the non 'x' aware cell.  For example,
+   during synthesis `$eqx` will typically be treated as `$eq`.
+
+.. cell:defprop:: x-output
+
+   These cells can produce 'x' output even if all inputs are defined.  For
+   example, a `$div` cell with ``B=0`` has undefined output.
+
+Refer to the :ref:`propindex` for the list of cells with a given property.