docs(am): add openapi specification

Author: remisultan

am/current/management-api/3.21/index.html

@@ -0,0 +1,10 @@
+---
+layout: api
+---
+
+<redoc
+  spec-url='{{ "swagger.json" }}'
+  native-scrollbars="true"
+  scroll-y-offset="50"
+  min-character-length-to-init-search="1"
+></redoc>

am/current/management-api/3.21/swagger.json

@@ -11,6 +11,7 @@ AM API exposes a complete Restful API accessible to anyone wanting to script som
 
 You can access the online API reference or the OpenAPI specification:
 
+* 3.21 -- link:{{ '/am/current/management-api/3.21/index.html' | relative_url }}[online reference] - link:{% link /am/current/management-api/3.21/swagger.json %}[OpenAPI spec]
 * 3.20 -- link:{{ '/am/current/management-api/3.20/index.html' | relative_url }}[online reference] - link:{% link /am/current/management-api/3.20/swagger.json %}[OpenAPI spec]
 * 3.19 -- link:{{ '/am/current/management-api/3.19/index.html' | relative_url }}[online reference] - link:{% link /am/current/management-api/3.19/swagger.json %}[OpenAPI spec]
 * 3.18 -- link:{{ '/am/current/management-api/3.18/index.html' | relative_url }}[online reference] - link:{% link /am/current/management-api/3.18/swagger.json %}[OpenAPI spec]