Help: Limit reference manual toctree depth to 1

Add a :maxdepth: option to all toctree directives to limit their depth
to 1.  We do not want subheadings from individual documents to pollute
the already large lists of CMake Domain objects.
Merge topic 'doc-manual-toctree-depth' into next

c76e44e Help: Limit reference manual toctree depth to 1