Add an integrity-types explanation page (the seven integrity types) #217

The DataJoint book (20-concepts/04-integrity.md) organizes the whole data model around seven integrity types — domain, completeness, entity, referential, compositional, consistency, workflow — each mapped to a concrete table feature, plus a real-world-rule → database-constraint mapping. It's the book's single most structuring idea.

The docs cover entity / referential / compositional integrity only in pieces (spread across explanation/entity-integrity.md and reference/specs/master-part.md), and domain, completeness, consistency, and workflow integrity are never named as integrity types. There is no page that enumerates the seven.

Proposal: add explanation/data-integrity.md enumerating all seven integrity types with the rules→constraints table, and cross-link the existing entity/referential/compositional material into it (and from explanation/index.md).

From an internal datajoint-book → docs coverage review (priority P1).