Refactored documentation

My first pass at refactoring documentation into a few different files. I've also spent some time simplifying the configuration and removing the exception thrown when no client or index is defined - making it simpler for new users to get started with the bundle.

I'd like some feedback regarding this one specifically from anyone who uses the bundle. I'd like to know if I've missed anything - or if there are features available in the bundle that were not originally documented.

You can see a rendered copy of the documentation here: https://github.com/merk/FOSElasticaBundle/tree/documentation

- [ ] Fix Features list on README.md
- [ ] Go over all changes since 2.1.x that need to be listed in UPGRADING.md