Milestone m4: user documentation content

[CooperBigFoot]

Integration PR for milestone m4 of the pyshed-release-and-docs-modernization campaign.

• Home, Quickstart (install → Engine over the hosted GRIT HFX dataset → delineate → area_km2/GeoJSON), Datasets guide, Staged-API guide (resolve_outlet → traverse → pre_merge_units → refine → dissolve → compose_result), and API Reference intro prose around the static mkdocstrings directive.
• Content triage: basin-geoparquet-export.md intro reframed for a public audience; raster-cache.md rewritten around current 0.3.0 refinement semantics (manifest-first D8 collapse; refine=False as escape hatch).

All snippets verified against the .pyi surface; strict docs build green with no compiled extension or GDAL. Covers vision items 11-12 (criterion 4 Quickstart/API portions). One squashed step commit (#18).