docs(method-guide): [T199] add method_guide.md decision-table router (#298)

New "Choosing a Method" page: per-domain decision tables mapping each data
situation/goal to the recommended estimator and its @ref page. Mutual
cross-links with getting_started.md. Registers the page in make.jl and the
index @contents. Draft make.jl build clean; no broken cross-references.

Claude-Session: https://claude.ai/code/session_01U961hhdMRdrcbNjmACaVVQ
