fix: allow passing guesses to linearize, improve linearization docs

[AayushSabharwal]

Close #3400

[AayushSabharwal]

The @time results:

julia> @time linearize(simplified_sys, linfun; op = Dict(x => 0.0));
  0.001373 seconds (416 allocations: 14.547 KiB)

julia> @time fast_linearize!(linprob, setter!, 1.0);
  0.000174 seconds (114 allocations: 4.844 KiB)