Reorganization of combinatorics documentation

[tscrim]

I will be separating out some of the larger subsections (crystals, symmetric functions, words, possibly more) into their own doc build folders for docbuilding speed. Also there will be better organization of the documentation within the combinatorics index.

Depends on #16058
Depends on #16256

CC: @sagetrac-sage-combinat @nthiery @nathanncohen @jpflori

Component: combinatorics

Author: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/14582

[nathanncohen]

Dependencies: #16058

[nthiery]

Changed dependencies from #16058 to #16058, #16256