Update codon models and frequencies documentation

[Artoria2e5]

Add documentation about codon model from a close reading of https://github.com/iqtree/iqtree3/blob/master/model/modelcodon.cpp.

The main change is a detailed note on how the mechanistic models compute rates. This is required because

• MG does not actually implement what MG1994 does, specifically deviating in how multi-nt changes are handled: the original paper sets a rate of 0 while iqtree does something else.
• The 1KT{S,V} and 2K models are very reasonable mechanistic approches to multi-nt changes, but people won't know they're reasonable unless we describe it.

I also added a mention of GY0K. It does not have a literature reference but is in the code. Describing it first makes describing all the GY-with-kappa models easier.

Other small changes are:

• Shortened the parameter sentence, since it's too repetitive. We've already defined that dn/ds, ts, tv are, so just use them.
• ECM alias for ECMK07 because it is in the code.
• Mention the difference between MG and GY0K in the "substitution rates" section by pointing to the frequency section. Adding a pointer here helps answer questions like "what's the difference between MG2K and GY2K?"