Userguide setup: defining R(B/H)FE calculations

[IAlibay]

cc @hannahbaumann

The way things are currently laid out for these sections makes the information flow a little bit difficult for users - it splits attention between protocol specific docs, alchemical networks, and these pages.

It might be good to consider re-structuring things.

Some ideas:

1. Move the "planner" ideas into a "Alchemical Network Planners" page. This would solely list planners.
2. Move protocol specific ideas, i.e. "these are the specific alchemical network types this protocol needs" into the protocol specific docs
3. Merge any alchemical network bits into the alchemical networks setup page

[IAlibay]

This is complete I believe, someone re-open if that's not the case.