userguide: detailed multistate and structural analysis pages

[IAlibay]

cc @hannahbaumann

Something worth dicussing. Given that several protocols use the same analysis methods, and that it comes with nice images etc... it might be good to have a userguide page specific to these analyses that offers in depth breakdowns of what analyses are provided, with images and references.

This could then be linked to by different Protocols without duplication of effort.

[IAlibay]

Re-opening as incomplete: see https://github.com/OpenFreeEnergy/openfe/blob/main/docs/guide/protocols/absolutesolvation.rst?plain=1#L78