adding google tag manager tracking code

Author: BlackstoneEngineering

antora/antora-playbook.yml

@@ -1,6 +1,10 @@
 site:
   title: RISC-V Ratified Specifications Library
-    #url: https://docs.riscv.org
+  # url: https://developer.riscv.org
+  keys:
+    google_analytics: 'GTM-NXLCGZF4'
+
+    
 
 output:
   dir: ./build/docs/reference