Explanatory reference layer around the ServiceAccess documentation
Adds 21 pages that explain why and how, for a colleague returning to the
project: The pipeline in plain words (a toy area walked through every stage),
Branches data and environment (state of play at e25416b), the method pages
(facility location model, travel cost functions, lambda sweep and Pareto
frontier, rounding, scenarios S1/S2/S3, scope rules, metrics, background
theory), the code pages (GeoDMS OD matrix and choice set, walkthroughs of
settings.jl / lp_run.jl / lambda_sweep_simplex.jl, running a sweep, log lines
and output files, deck pipeline) and the reference pages (decision log,
alternatives not pursued, FAQ, glossary). New Home with a reading order and
a sidebar. Four explainer figures with their generator (scripts/make_figures.py).
Maarten's formal pages are unchanged apart from three marked editor's notes
where they lag the code at e25416b (grid extension removed in f93aea1;
S1/S2 pinned by bisection since #52; the NL numbers in results §6 are the
snapped row) and a notation warning on the school-era page.