Add a section on front controllers and the AppKernel

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | n/a

Here are some additions on how front controllers and custom AppKernels can be used to further tweak the runtime environment.

This is my first piece of .rst, so probably a lot of things and markup needs to be fixed. Additionally, we should discuss which is the best place for this document.