Document preconfigured eager loading

[elpete]

Closes #47

Issue review

Recommendation: 9/10 — document. Preconfigured eager loading is powerful but currently discoverable only by reading implementation or tests. Documenting its syntax, nested paths, query coverage, and over-fetching tradeoff makes the feature safer to adopt.

Documentation

• shows how to configure variables._with on an entity
• includes normal and nested relationship paths
• explains that preconfiguration is applied to new entity queries, including all, get, first, and find
• recommends explicit query-level eager loading for relationships not needed by most callers
• warns that automatic eager loading adds queries and can over-fetch data

Repository note

The standalone cbQuickDocs repository currently exposes only a master branch. Per the requirement that every PR in this issue pass target next, this documentation is added to Quick’s repository README and the PR targets next.

Validation

• git diff --check
• no runtime tests run; documentation-only change

The documented behavior is covered by the existing automatic eager-loading integration spec. Uses qb@14.0.0-beta.3.