RTDs: Fail on warning

Fix outstanding sphinx warnings;
- Change mycells.lib to use `text` parsing instead of (currently) unsupported `Liberty`.
- Remove unused `troubleshooting.rst`, moving the todo into the index.

docs/source/using_yosys/more_scripting/index.rst

@@ -3,6 +3,8 @@ More scripting
 
 .. todo:: brief overview for the more scripting index
 
+.. todo:: troubleshooting document(?)
+
 .. toctree::
    :maxdepth: 3