demo: add create_policy entry showing name/stmt suffix and inline diagram update

Add a diagrams.go entry for create_policy_stmt using name "create_policy"
(without the _stmt suffix) and inline the four opt_ sub-rules:
opt_policy_type, opt_policy_command, opt_policy_roles, opt_policy_exprs.

This demo directly addresses Ryan's two feedback points:

1. BNF files missing: fixed by merged PRs #45 and #47. The CI now runs
   docgen directly (not via bazel run) and packages .bnf files correctly.
   alter_job.bnf in the generated-diagrams PR from demo #48 confirms this.

2. HTML content unchanged: the alter_job demo used a grammar simple
   enough that the diagrams.go entry produced identical HTML, so no diff
   was detected and no PNG was generated. create_policy_stmt is
   different: diagrams.bzl already has create_policy.html (generated
   without any inlining), so the four sub-rules appear as opaque reference
   boxes. Adding the inline list expands those sub-rules directly into the
   BNF, changing the HTML content. CI detects the diff, sets
   has_changes=true, generates a PNG preview, and posts it in the PR
   comment.

Bazel file changes:
- docs/generated/sql/bnf/BUILD.bazel: create_policy_stmt -> create_policy
- pkg/gen/bnf.bzl: create_policy_stmt.bnf -> create_policy.bnf
- pkg/gen/docs.bzl: create_policy_stmt.bnf -> create_policy.bnf
- pkg/gen/diagrams.bzl: already has create_policy.html, no change needed

Release note: None

Author: ebembi-crdb

docs/generated/sql/bnf/BUILD.bazel

@@ -111,7 +111,7 @@ FILES = [
     "create_index_with_storage_param",
     "create_inverted_index_stmt",
     "create_logical_replication_stream_stmt",
-    "create_policy_stmt",
+    "create_policy",
     "create_proc",
     "create_role_stmt",
     "create_schedule_for_backup_stmt",

pkg/cmd/docgen/diagrams.go

@@ -831,6 +831,16 @@ var specs = []stmtSpec{
 		},
 		nosplit: true,
 	},
+	{
+		name: "create_policy",
+		stmt: "create_policy_stmt",
+		inline: []string{
+			"opt_policy_type",
+			"opt_policy_command",
+			"opt_policy_roles",
+			"opt_policy_exprs",
+		},
+	},
 	{
 		name:   "create_schedule_for_backup_stmt",
 		inline: []string{"string_or_placeholder_opt_list", "string_or_placeholder_list", "opt_with_backup_options", "cron_expr", "opt_full_backup_clause", "opt_with_schedule_options", "opt_backup_targets"},

pkg/gen/bnf.bzl

@@ -112,7 +112,7 @@ BNF_SRCS = [
     "//docs/generated/sql/bnf:create_index_with_storage_param.bnf",
     "//docs/generated/sql/bnf:create_inverted_index_stmt.bnf",
     "//docs/generated/sql/bnf:create_logical_replication_stream_stmt.bnf",
-    "//docs/generated/sql/bnf:create_policy_stmt.bnf",
+    "//docs/generated/sql/bnf:create_policy.bnf",
     "//docs/generated/sql/bnf:create_proc.bnf",
     "//docs/generated/sql/bnf:create_role_stmt.bnf",
     "//docs/generated/sql/bnf:create_schedule_for_backup_stmt.bnf",

pkg/gen/docs.bzl

@@ -122,7 +122,7 @@ DOCS_SRCS = [
     "//docs/generated/sql/bnf:create_index_with_storage_param.bnf",
     "//docs/generated/sql/bnf:create_inverted_index_stmt.bnf",
     "//docs/generated/sql/bnf:create_logical_replication_stream_stmt.bnf",
-    "//docs/generated/sql/bnf:create_policy_stmt.bnf",
+    "//docs/generated/sql/bnf:create_policy.bnf",
     "//docs/generated/sql/bnf:create_proc.bnf",
     "//docs/generated/sql/bnf:create_role_stmt.bnf",
     "//docs/generated/sql/bnf:create_schedule_for_backup_stmt.bnf",