User-facing instructions for inorganics

[JanCBrammer]

Add documentation to support users who use the inorganics feature:

• explanation of common warnings
• how to best use Ketcher with inorganic structures
  • for donor atoms beyond N please change the “valence” (i.e., coordination number) of the atoms accordingly by right-clicking on the corresponding atoms
• input crystal structures as molfile (convert cif to a molfile via Mercury)
• note bridging H atoms and H atoms at metals explicitly to save coordination environment