[Request] Rename some documents

[toby63]

I recommend renaming some documents to have a better structure and readability.

I would like to change the following names:

• folder: docs/devops-guide to self-hosting-guide or server-guide
• devops-guide.md to self-hosting-guide or server-guide
• secure-domain.md to access control or something similar
• quickstart.md to debian guide or debian/ubuntu guide

I would like to change the ids as well, because their names are also a bit confusing.

We would need to change all the links (internal and external) as well to fit the new structure.
In addition it might be possible to redirect the old links for some time, to guarantee accessability.

[saghul]

There is a way to keep old ids around. We should rename the devops parts to self-hosting, but now that this is our id like to keep old links around.