cellref: Deprecate cell_library.rst

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`.

docs/source/cell/word_formal.rst

@@ -0,0 +1,14 @@
+Formal verification cells
+-------------------------
+
+.. TODO:: Describe formal cells
+
+   `$check`, `$assert`, `$assume`, `$live`, `$fair`, `$cover`, `$equiv`,
+   `$initstate`, `$anyconst`, `$anyseq`, `$anyinit`, `$allconst`, and `$allseq`.
+
+   Also `$ff` and `$_FF_` cells.
+
+.. autocellgroup:: formal
+   :members:
+   :source:
+   :linenos: