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

[MrGeneration]

To help with the transition, especially when the docs are replaced (which applies to user and admin docs as well in my opinion), we should redirect all URLs. Because we have those URLs in place virtually everywhere and technically for a nginx vhost it's zero effort to handle that.

It's better user experience and refrains us from dead links.