Add using the rest api by referring to security/api.rst (#49675)

(cherry picked from commit 7077bdf)

Author: rawwar

airflow-core/docs/howto/index.rst

@@ -30,6 +30,7 @@ configuring an Airflow environment.
     :maxdepth: 2
 
     Using the CLI <usage-cli>
+    Using the REST API <../security/api>
     add-dag-tags
     notifications
     set-config