Crystal and henry docs

[Surluson]

Added markdown docs for henry.jl and crystal.jl

[SimonEnsemble]

thanks Arni! can u plz add:

• description of how to infer bonds and how that's done
• discuss P1 symmetry vs not
• for henry coeff. maybe say what henry's law is in math mode: m = H p

some of this is here in the old docs. might be good to check these and mk sure you are not leaving anything out. https://simonensemble.github.io/PorousMaterials.jl/stable/manual/boxes_crystals_grids/

[SimonEnsemble]

@Surluson going to merge this, plz make a separate PR with the additions