docs: add 2D Grain Structure Geometrification page; retire rasterio docs
Why: no wiki page explained geometrification -- what
polygonised_grain_structure (Technique A) and GrainManifold2D
(Technique B) do, how each handles island grains, or how to choose
between them -- despite it being covered only obliquely, by bare
notebook filenames, on Meshing.md and Demo-Notebooks.md. Separately,
Data-IO.md and four other pages described pxtal_ori_map_2d.py as a
rasterio consumer and the [io] extra as still needed; both are now
false (see the upxo_library rasterio-retirement commit).
What:
- New page 2D-Grain-Structure-Geometrification.md: both techniques'
pipelines (four programmatically alignment-verified ASCII diagrams,
not hand-typed -- pipeline position, Technique A stages, Technique B
stages, island label-grid plus resulting polygons), island handling
in each, raster_polygonize/raster_islands, a property comparison
table (no hardcoded performance numbers -- points at
confMesh2d_compare_techniques.ipynb instead, for the same reason its
own stale numbers needed fixing), demo notebooks, source map, status
notes. Linked from Meshing.md, Demo-Notebooks.md,
2D-Grain-Structure-Generation.md (as pipeline step 4), Architecture.md,
Home.md, and _Sidebar.md.
- Demo-Notebooks.md, Meshing.md: added confMesh2d_compare_techniques.ipynb,
missing from both notebook catalogues.
- Data-IO.md, Architecture.md, Dependencies.md, FAQ.md, Home.md: rasterio
is not a UPXO dependency; the [io] extra and every rasterio row/mention
removed, not just narrowed to "one legacy script".
Verified: every internal link target is a real page; grepped the whole
repo afterward for "rasterio" -- the only hits left are factual,
past-tense statements that it is no longer a dependency.