docs: add data flow page to navigation

Adds the new data flow documentation page to the Security and
privacy section in the docs navigation.

VAP-11220

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

Author: vapi-tasker[bot]

fern/docs.yml

@@ -682,6 +682,8 @@ navigation:
             icon: fa-light fa-shield-check
             collapsed: true
             contents:
+              - page: Data flow
+                path: security-and-privacy/data-flow.mdx
               - page: JWT authentication
                 path: customization/jwt-authentication.mdx
               - page: Recording consent plan