Reorganize reference manual chapter "discrete geometry"

[mkoeppe]

• Convex sets
• Convex polyhedra
• Lattice polyhedra
• Combinatorial polyhedra
• Arrangements, dissections, complexes

Depends on #25675

CC: @jplab @kliem @yuan-zhou

Component: documentation

Issue created by migration from https://trac.sagemath.org/ticket/34194

[mkoeppe]

Dependencies: #25675