[WIP] Toctrees and maps as done in the core docs

This PR tries to make a consistent site, where both the core and the cmf docs use the same way of creating toctrees and maps.

This also means we can go to http://symfony.com/doc/master/cmf/bundles/ and get a list of all bundles.

The map of a directory is placed in a `map.rst.inc` file in that directory. This is the list of articles that is shown on the site. Each directory got also an `index.rst` file. The `map.rst.inc` file is included here, and there is a hidden toctree (because sphinx wants all files to be included in a toctree).

On the main `index.rst` file, there is a toctree for the sub `index.rst` file and we include the `map.rst.inc` file again.

### Questions

I've 2 questions:

 1. The doc links in `map.rst.inc` are difficult because we use relative links. Is there a reason why you use relatve links?
 2. On the core docs, we made the ``map.rst.inc` of the components somewhat more than just a list of links. Do you want me to do that with the bundles in the cmf docs?