docs: Add PostGIS geography fields documentation

[charlesarchibong]

Summary

• Add a new Geography fields page documenting the four supported types: GeographyPoint, GeographyLineString, GeographyPolygon, and GeographyGeometryCollection, with YAML model examples for each.
• Add a Geography operators section to the filter docs covering intersects, dwithin, distance, contains, and within, with Dart code examples.
• Add a Geography indexes section to the indexing docs explaining GIST index setup for geography columns.
• Update the supported types list in the models overview to include all four geography types.
• Add a new Upgrading to PostGIS support guide covering Docker-based environments (including a combined pgvector + PostGIS Dockerfile), managed PostgreSQL services, and troubleshooting tips.

Test plan

• Verify all new pages render correctly in Docusaurus with no broken links.
• Confirm cross-links between geography fields, indexing, filter, and upgrade guide pages resolve correctly.
• Check that the new upgrade guide appears in the correct position under the Upgrading section (09-upgrading).