Generate index.rst files

[ryanmrichard]

If you run cmakedoc -o docs/source/api -r cmake in the top-level of the CMakePPCore repo you get a series of directories:

.../api/cmakepp_core .../api/cmakepp_core/object etc.

Each of these holds documentation files for the various functions etc., which is perfect. However, to actually include these files in the main documentation one needs index.rst files in docs/source/api as well as in all subdirectories. These index.rst files should be autogenerated so that all a user has to do is add docs/source/api to their existing index.rst file in docs/source.