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/v9.20.18/grammar/rndc.grammar

@@ -0,0 +1,20 @@
+key <string> {
+	algorithm <string>;
+	secret <string>;
+}; // may occur multiple times
+
+options {
+	default-key <string>;
+	default-port <integer>;
+	default-server <string>;
+	default-source-address ( <ipv4_address> | * );
+	default-source-address-v6 ( <ipv6_address> | * );
+};
+
+server <string> {
+	addresses { ( <quoted_string> [ port <integer> ] | <ipv4_address> [ port <integer> ] | <ipv6_address> [ port <integer> ] ); ... };
+	key <string>;
+	port <integer>;
+	source-address ( <ipv4_address> | * );
+	source-address-v6 ( <ipv6_address> | * );
+}; // may occur multiple times