docs/conf.py: Include 3 levels of ToC in latexpdf output.

Instead of default 2. 3 are required to access description of individual
library modules.

docs/conf.py

@@ -246,6 +246,8 @@ latex_elements = {
 
 # Additional stuff for the LaTeX preamble.
 #'preamble': '',
+# Include 3 levels of headers in PDF ToC
+'preamble': '\setcounter{tocdepth}{2}',
 }
 
 # Grouping the document tree into LaTeX files. List of tuples