Make parts of documentation optional

[michakraus]

Some examples, in particular the Toda Lattice animation, take a long time to compute.

We should add a flag and Makefile entry to skip those parts.

[michakraus]

Proposed fix (for when we pick this up):

• In docs/make.jl, gate slow/animation examples behind an env var, e.g. build_animations = get(ENV, "GEOMETRICPROBLEMS_DOCS_FULL", "false") == "true", and skip the corresponding pages/blocks when false.
• Convert the Toda animation block in docs/src/toda_lattice.md to an @example block that is only executed when the flag is set (or keep it a plain julia block and add a documented make target).
• Add a docs/Makefile target (e.g. documenter-full:) that sets the env var, leaving the default documenter: target fast.
• Reuse the existing CI-detection idiom already in make.jl (haskey(ENV, "CI")).

Leaving open; not implementing yet.