docs: update documentation to OKF v0.1 format

docs/database/challenges.md

@@ -56,6 +56,14 @@ canonical enums, the `enabled` flag, `updated_at`, the
 category FK stays at `ON DELETE RESTRICT` so the
 `CATEGORY_HAS_CHALLENGES` business rule is preserved.
 
+The `SeedSampleChallenges1700000000600` migration inserts a small
+representative set of sample challenges (one or more per canonical
+system category) on a fresh database so the `/challenges` board has
+cards immediately after bootstrap. It is idempotent: if the
+`challenge` table already contains rows (e.g. an admin imported a
+custom challenge set), it does nothing. Sample flags follow the
+`flag{<token>}` convention and are exposed only inside admin views.
+
 ## `challenge_file`
 
 | Column              | Type    | Description                          |