Fix .rst doc files, auto-generation script

Our documentation is produced by reading the set of .rst files in `docs/source/api` - these files dictate which functions and classes are documented where, with what style, and how documentation is grouped into chapters.

Maintaining these rst files is a pain, so I've written a little script for generating them. To use it, ensure menpo is in your Python interpreter, then
```
cd docs
python generate_rst.py
```
Will regenerate the rst files from the Menpo source in the style we like. In general, this will need to be run in the future when we add, remove, rename or move any class or function in Menpo. Don't just blindly commit the results though - run it, and then update the relevant index.rst file in the package to ensure everything makes sense and run a build.

This PR includes updated .rst files based on the auto-generation with some hand crafted changes.