Refactor documentation - split into different roles

We could refactor the documention, by having 3 seperate doc pages, which explain schleuder from the different roles of a user:

• simple list user / subscription
• list admin
• schleuder server admin

This would allow to give role specific introductions, as well as being able to address specific use-cases a user would want to do in a certain role.

Hopefully, this will make the documentation more appealing and easier to understand, while it could even go further into details without overloading a single page with too much documentation.