Cantera 3.0 website coverage-dependent surface species documentation

[ischoegl]

Problem description

Documentation of coverage-dependent-surface species thermo is confusing.

Unlike other entries that (usually) correspond to a single C++ class/struct, this entry covers four different species coverage models (see doxygen and source code). In #1548 (now closed), this was addressed by a separate entry "Species Coverage Models", see proposed MarkDown (same level as "Species Critical State Parameters")