Phase out of system docs

[ralf401]

Now that most content is migrated, it makes sense to think about when and how to phase the current system documentation out.

We should not wait until the user and admin content is created because this will be done in parallel to the desktop view.

Open questions:

• When should we do that?
• Ok to first put a hint in the system documentation with a link to the new one?
• Landing page in new docs which explains the current state?
• Forwarding of URLs needed? If yes, which one?
• TBD