docs: add variable substitution page to Chat navigation

VAP-11219

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vapi-tasker[bot]

fern/docs.yml

@@ -479,6 +479,9 @@ navigation:
           - page: Session management
             path: chat/session-management.mdx
             icon: fa-light fa-layer-group
+          - page: Variable substitution
+            path: chat/variable-substitution.mdx
+            icon: fa-light fa-brackets-curly
           - page: SMS chat
             path: chat/sms-chat.mdx
             icon: fa-light fa-comment-sms