docs(site): write user documentation content

[CooperBigFoot]

User documentation content: Home, Quickstart, guides, API Reference, triage (m4)

Fills the placeholder pages m3 created with real user-facing prose and triages two
existing docs. Content-only — mkdocs.yml and the mkdocstrings directive config
are untouched (m3-owned); how-it-works.md and credits.md are untouched
(m5-owned).

What's here

• docs/index.md — Home/overview: what shed/pyshed is, who it's for, install
  one-liner, links into the nav.
• docs/quickstart.md — install → open an Engine against the hosted GRIT
  HFX dataset (zero-download) → delineate a (lat, lon) → read area_km2 and
  to_geojson(), in one copy-pasteable script; outlet / delineation / HFX-dataset
  glossed.
• docs/guide/datasets.md — the HFX artifacts (manifest.json,
  catchments.parquet, graph.parquet, optional snap.parquet + D8 rasters),
  the canonical hosted GRIT dataset (URL + Upstream Tech hosting attribution), and
  local vs. remote opening.
• docs/guide/staged-api.md — the staged pipeline select_level → resolve_outlet → traverse → pre_merge_units → refine → dissolve → compose_result, the typed object each stage returns, and staged vs. one-shot
  guidance.
• docs/api-reference.md — new intro prose around m3's unchanged ::: pyshed
  directive.
• docs/basin-geoparquet-export.md — reframes the internal "Status" /
  "Elevation Path" framing for a public audience; schema and technical body
  preserved; not elevated into a versioned spec.
• docs/raster-cache.md — trimmed toward the user-relevant refine=False
  caveat.

Verification

• uv run --project crates/python --only-group docs mkdocs build --strict
  succeeds with zero warnings and no cargo/maturin/GDAL activity (no
  target/).
• Static autodoc still renders Engine and DelineationResult.
• Every Python snippet matches the shipped __init__.pyi signatures.

Expected CI behavior (not a regression)

• This PR edits only Markdown under docs/, so it does not trigger the
  cibuildwheel/GDAL wheel build (that path filters on crates/python/**, not
  docs/**). docs.yml runs on push-main, not on PRs; its deploy job stays
  human-gated on Pages enablement.