GEAR documentation improvement
This is not ready to be merged.
In this merge request, I updated GEAR documentation:
- I reorganized sections and created new ones. This will allow the consistent and easy integration of our future modules into the documentation.
 - The sinks are now within the GEAR pages
 - I made some improvements here and there to consider the recent developments.
 - I moved and updated the snapshot outputs section to a more appropriate location
 
@yvesrevaz, can you check and tell me if it sounds good? Moreover, the feedback/stellar evolution part deserves an update if you have any ideas or comments.