Added interlinking and fixed install template for reusable bundles

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixet tickets | -

See commit descriptions for more info
There were some more style issues and I ink even some images weren't loaded. But this seems to be fixed: http://symfony.com/doc/current/best_practices/introduction.html