BC-11238 - Adjust documentation of BBB

[MartinSchuhmacher]

Description

Following the movement of the documentation of BBB in
#81
we want to bring this documentation to a recent and more up-to-date state.

Links to Tickets or other pull requests

https://ticketsystem.dbildungscloud.de/browse/BC-11238

Screenshots of UI changes

Approval for review

• DEV: If the API or client code was changed, all necessary code generation or synchronization steps were completed and tested.
• QA: In addition to review, the code has been manually tested (if manual testing is possible)
• All points were discussed with the ticket creator, support-team or product owner. The code upholds all quality guidelines from the PR-template.