Fix missing RowsAffected checks in cookie update methods

Address PR review comments: capture Exec result and check
RowsAffected() == 0 to return ErrResourceNotFound in
Cookie.Update, CookieCategory.Update, and CookieCategory.UpdateRank.
Also update coredata and relay contributor docs accordingly.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/coredata/cookie.go

@@ -327,7 +327,7 @@ WHERE
 	}
 	maps.Copy(args, scope.SQLArguments())
 
-	_, err := tx.Exec(ctx, q, args)
+	result, err := tx.Exec(ctx, q, args)
 	if err != nil {
 		if pgErr, ok := errors.AsType[*pgconn.PgError](err); ok {
 			if pgErr.Code == "23505" && pgErr.ConstraintName == "idx_cookies_unique_name_per_banner" {
@@ -337,6 +337,10 @@ WHERE
 		return fmt.Errorf("cannot update cookie: %w", err)
 	}
 
+	if result.RowsAffected() == 0 {
+		return ErrResourceNotFound
+	}
+
 	return nil
 }