MAINT: optimize: reorganize the list of optimization routines

When new users come to scipy.optimize, they may be confused by the slew of minimizer routines offered; I was when I did recently, and I think it's visible on the mailing list as well.

This attempts to reorganize the list of minimizers to better promote the new minimize(), minimize_scalar() interfaces (the fmin* interfaces are listed only after a note not recommending to use them) and split the functions primarily as local vs. global minimizers.

(Possibly, one could just say "deprecated" instead of "not recommended to use in new code", but I'm not brave enough to do that myself, or sure that's 100% the plan.)

(I'm also not sure whether not to just put Equation (Local) Minimizers in the Fitting section. I don't care personally, I think both approaches work.)