feat: add efcore-d2-db-diagram skill and related documentation (#1821)

skills/efcore-d2-db-diagram/references/grouping-modes.md

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+# Grouping Modes
+
+## bounded-context
+
+Group tables by domain area using folder, namespace and naming clues.
+
+Examples:
+
+- Clients
+- Offers
+- Freelances
+- Billing
+- Audit
+- Identity
+
+## schema
+
+Group by database schema from `ToTable` or migrations.
+
+## namespace
+
+Group by C# namespace.
+
+## flat
+
+Do not create containers.
+
+Use flat mode for small schemas or when the user wants maximum compatibility.