Consolidate contributor editorial guidance

[BLCCoreStudio]

Consolidates the still-useful documentation from stale/conflicting PRs #90-#94 onto current main.

Changes:

• add focused contribution paths for corrections, sources, examples, architecture, relationships, deepening, and discovery;
• add source-quality guidance with claim-level evidence/freshness discipline;
• add kind-specific editorial review checklists;
• add relationship suggestion/validation guidance;
• add deterministic generated index/search preview guidance;
• link all five guides from CONTRIBUTING.md;
• fix the stale generated-preview filename reference while consolidating.

This is documentation-only and intentionally does not change schemas, catalogs, graph data, ranking, or publication behavior.

Supersedes #90, #91, #92, #93, and #94.