docs: document restricted diagram operations (new in 2.2)

[dimitri-yatsenko]

Summary

Document the graph-driven diagram operations introduced in DataJoint 2.2. The Diagram is an inspection and graph computation tool — all mutation (delete, drop) is executed by Table.delete() and Table.drop().

Changes

• diagram.md: Add operational methods section documenting cascade(), restrict(), preview(), prune() with restriction propagation rules, OR-vs-AND convergence semantics, and part_integrity behavior. No delete() or drop() on Diagram.
• whats-new-22.md: Add graph-driven diagram operations section covering motivation, preview-then-execute pattern (Diagram for inspection → Table.delete() for execution), two propagation modes, architecture, and comparison table vs error-driven cascade.
• delete-data.md: Add "Inspecting Cascade Before Deleting" section showing delete(dry_run=True) for quick preview and dj.Diagram(schema).cascade().preview() for detailed inspection. Deletion always through Table.delete().
• data-manipulation.md: Version admonition noting graph-driven cascade internals in 2.2.
• master-part.md: Document part_integrity="cascade" upward propagation behavior.
• Cross-references between all affected files.

Key design decision

Diagram has no delete() or drop() methods. It provides cascade() and preview() for understanding the blast radius; Table.delete() handles transactions, SQL execution, prompts, and error handling. This keeps the Diagram as a pure graph computation tool.

Test plan

• mkdocs build succeeds with no new warnings
• Review rendered pages for diagram.md, whats-new-22.md, delete-data.md
• Verify cross-reference links resolve correctly

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