Documented new site architecture

- Added section on bower
- Added section on documentation:
  - how to update
  - tools required for rendering
Added `documentation` target to `Makefile`

Added two targets to `Makefile`:

- `composer`, which will perform a `self-update`.
- `documentation`, which depends on `composer`, and which updates the
  `zfcampus/apigility-documentation` package.

Updated the README to indicate that `composer.phar` and `composer.lock` get
updated on a `make all`, and to use `make documentation` to update the docs.