Rework sectioning in mutations page

[jeromekelleher]

The sectioning for the mutations page isn't working very well I think, and needs a bit of work. In particular, there's an awful lot of stuff under the "model" section, which I think is unhelpful. We should probably move the documentation of the general matrix mutation model stuff to it's own top level section, and likewise for all the other types of models like microsats. If someone wants to simulate microsats they should be able to see this from the top-level nav on the right.