Add "advanced fixtures" topic to docs

[nicoddemus]

We should add a "advanced topics" chapter to the documentation, possibly:

• Invocation-scoped fixtures (invocation-scoped fixtures #1681)
• scope defined by a callable (Use a callable to define fixture scoped dynamically #1682)

[The-Compiler]

Since I changed the milestone for #1682 to 3.1 and we have some basic docs for invocation-scoped fixtures, I'm changing the milestone here too.

[hackebrot]

Hmm given that the referenced features have yet to be added, I wonder if it makes sense to wait for the according pull requests before we add the section to the docs.

[nicoddemus]

I think we can close this given that we are refactoring the entire documentation structure.