chore: bump gem version to v0.21.0

See merge request gitlab-org/glql!330

Add documentation philosophy section establishing guidelines for keeping
AGENTS.md lean and focused. Extract detailed references and procedural
guides to doc/ directory to minimize context usage for AI agents while
maintaining accessibility.

Co-authored-by: James Louw <jlouw@gitlab.com>

- Add naming conventions for GLQL fields and type values
- Clarify nested resource handling with flat response requirements
- Document higher-level access patterns for optional parent filters
- Update enum case mapping guidance for API inconsistencies
- Add resource-specific filter/field mapping guidance
- Improve scoping flexibility documentation with examples

docs: improve GraphQL data source architecture guide

See merge request gitlab-org/glql!328

chore: update gem release reviewers

See merge request gitlab-org/glql!331

docs: add documentation philosophy to AGENTS.md

See merge request gitlab-org/glql!327

Fix native gem builds

See merge request gitlab-org/glql!332

chore: bump gem version to v0.22.0

See merge request gitlab-org/glql!333