Add working group overview page

[codingfabi]

We should create a working group overview page that has the following functionalities:

• List all working groups
• Possibility to request to join a working group via button (blocked by Endpoint to request to join a working group #101 )
• see details of a working group -> some kind of profile page

In my opinion, this should be done in a table.

[codingfabi]

Endpoint is implemented, add frontend support asap.