docs: Add BIND9 grammar files for v9.18.44 and v9.20.18

- Fetch official BIND9 configuration grammar files from Gitea mirror
- v9.18.44: Complete grammar including delegation-only.zoneopt
- v9.20.18: Grammar files with v9.20 specific syntax updates
- Files organized in upstream/vX.Y.Z/grammar/ directory structure
- Enables configuration validation and version comparison tooling
- Supports documentation generation and compatibility checking

bind9-grammar/upstream/v9.18.44/grammar/static-stub.zoneopt

@@ -0,0 +1,13 @@
+zone <string> [ <class> ] {
+	type static-stub;
+	allow-query { <address_match_element>; ... };
+	allow-query-on { <address_match_element>; ... };
+	forward ( first | only );
+	forwarders [ port <integer> ]  { ( <ipv4_address> | <ipv6_address> ) [ port <integer> ]; ... };
+	max-records <integer>;
+	max-records-per-type <integer>;
+	max-types-per-name <integer>;
+	server-addresses { ( <ipv4_address> | <ipv6_address> ); ... };
+	server-names { <string>; ... };
+	zone-statistics ( full | terse | none | <boolean> );
+};