docs(graphile-postgis): deep spatial-relations docs; lead with client-side GeoJSON problem

[pyramation]

Summary

Documentation-only rewrite of graphile/graphile-postgis/README.md to go deep on the @spatialRelation smart-tag feature and lead with why it exists.

• New "The problem" section at the top, in plain English: client-side GeoJSON juggling → auto-generated ORMs don't know about spatial joins → we added the spatial-relation primitive so the join runs server-side.
• Rewrote the ## Spatial relations via smart tags section with:
  • At a glance: before vs after snippet contrasting client-side point-in-polygon against a single where: { county: { some: … } } query.
  • Declaring a relation — tag grammar, target ref resolution, multiple tags on one column.
  • Operator reference table covering all 8 operators with parametric / symmetric / directional guidance.
  • Using the generated where shape — ORM and GraphQL examples, some / every / none semantics, st_dwithin + distance, composition with and / or / not.
  • Self-relations, Generated SQL shape, Indexing (with @spatialRelationSkipIndexCheck opt-out), geometry vs geography, and FAQ.
• Terminology pass: user-facing verb is where; the word "filter" only appears where it literally names the PostGraphile-generated connection argument.
• All code examples are sourced from the integration test (graphql/orm-test/__tests__/postgis-spatial-relations.test.ts) and its seed SQL.

No code changes to the plugin, tests, or any other package.

Review & Testing Checklist for Human

• Spot-check the technical claims against graphile/graphile-postgis/src/plugins/spatial-relations.ts — especially the relationName regex, the existence / exact spelling of the @spatialRelationSkipIndexCheck opt-out tag, and the distance: Float! field shape.
• Confirm the "Multiple relations on one column" SQL snippet is a valid way to stack smart tags in a COMMENT ON COLUMN body (i.e. the plugin actually parses newline-separated @spatialRelation lines the way shown).
• Sanity-check the Operator reference symmetry / directionality column and the st_bbox_intersects → && note against the plugin's code generator.
• Re-read "The problem" section to make sure the voice matches how you'd describe this to a user.

Notes

• Docs-only diff; no build / lint / test changes were needed and none were run. CI will cover the usual static checks.
• Branch renamed mid-flight from the default devin/<ts>-… to feat/postgis-spatial-relations-deep-docs per the repo's branch-naming convention.

Link to Devin session: https://app.devin.ai/sessions/059b0c2e4594474aa107e3f83f83fe0f
Requested by: @pyramation

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