docs: explain grouped declarations

[kvlonge]

Why

Scoped declaration directories can blend into resource folders. SQLBuild now supports grouping them under a clearly identifiable _sqlbuild/ container.

Changes

• Document grouped public and private macro, enum, and constant paths.
• Explain owner and visibility behavior.
• Document compatibility with existing declaration paths.
• Explain reserved-container validation and project-root restrictions.
• Update placement and scope-explorer examples.

Verification

• Documentation diff review
• Cross-check against SQLBuild discovery, visibility, placement, and report behavior
• Public tree and outgoing-commit hygiene scans