docs(graphile-postgis): feature-led README with TOC and ORM examples per capability

[pyramation]

Summary

The graphile-postgis README had drifted: its top-level "The problem"
section was rewritten to describe only the spatial-relations use
case, erasing the generic PostGIS framing and leading with a problem
statement instead of the plugin's capabilities. This PR re-leads the
README with features, adds a table of contents, and documents every
capability with an ORM example where we have test coverage and a
GraphQL example otherwise — with spatial relations positioned as one
feature among many.

What changed:

• New feature-led intro + Table of contents jumping to every
  capability.
• Features at a glance checklist covering every plugin in the
  preset.
• One section per capability — each with a concrete usage example:
  • GeoJSON scalar and typed geometry columns — ORM
    select: { geom: { select: { geojson: true } } } (lifted from
    graphql/orm-test/__tests__/mega-query.test.ts).
  • Dimension-aware interfaces and subtype fields — subtype →
    fields table plus a GraphQL example.
  • Measurement fields (length / area / perimeter) —
    subtype → fields table plus GraphQL example, with a note that
    these are geodesic client-side helpers on top of GeoJSON, not a
    replacement for projection-aware SQL computed columns.
  • Transformation fields (centroid / bbox / numPoints) —
    GraphQL example, with guidance to use custom SQL functions for
    parametric ST_Transform / ST_Buffer / ST_Simplify /
    ST_MakeValid.
  • Per-column spatial filters — full taxonomy (topological /
    distance / 2D+ND bbox / directional bbox) with two ORM examples
    lifted from graphql/orm-test/__tests__/postgis-spatial.test.ts.
  • PostGIS aggregate fields (stExtent / stUnion /
    stCollect / stConvexHull) — GraphQL example.
  • Spatial relations — promoted to a top-level feature with a
    tight "Why a dedicated primitive" blurb, tag grammar + stacking,
    operator reference, ORM examples for some / every / none
    and parametric st_dwithin (lifted from
    graphql/orm-test/__tests__/postgis-spatial-relations.test.ts),
    a GraphQL example, composition, self-relations, generated SQL
    shape, indexing, geometry vs geography, and FAQ.
  • Graceful degradation — explicit section describing what
    happens when the extension is missing.

All ORM code samples are taken from live tests in
graphql/orm-test/__tests__/. GraphQL examples are used for fields
where we don't have dedicated ORM integration tests (dimension
accessors, measurement / transformation field selection, aggregate
fields) — all of which are unambiguously present in the generated
GraphQL schema via the plugins they ship with.

Docs-only; no source changes.

Review & Testing Checklist for Human

• Skim the new opening — does leading with features (instead of a
  problem statement) read the way you want it to?
• Scroll the TOC and confirm every anchor resolves to a real
  section on GitHub's rendered markdown.
• Sanity-check the Per-column spatial filters taxonomy — it's
  reconstructed from graphql/query/src/types/query.ts and
  graphql/orm-test/__tests__/postgis-spatial.test.ts; flag
  anything that shouldn't be surfaced here.
• Confirm the Spatial relations section still stands on its
  own as a feature with its own motivation paragraph (without
  taking over the whole README).

Notes

• Green-risk: docs-only, no code touched.
• Operator reference, composition, self-relations, generated SQL
  shape, indexing, geometry vs geography, and FAQ content are
  preserved verbatim from the previous version — only re-leveled
  under the new ## Spatial relations heading.

Link to Devin session: https://app.devin.ai/sessions/b1b7eb41d9634d289a6b8afadc051051
Requested by: @pyramation

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