✨(frontend) warn users when the connection falls back to TURN

Highlight in the connection test when the user is connecting
through a TURN relay, especially over TLS or TCP. This usually
indicates that some network configuration is required on their
side, and gives them a concrete signal to pass to their IT team.

Suggested by a technical user, this is a first step toward making
users more autonomous when troubleshooting access to the tool.

Follow-up: show a similar warning in-product when we detect a
mid-meeting fallback to TURN/TLS. A one-time hint for first-time
users would likely be enough.

src/backend/meet/settings.py

@@ -466,6 +466,11 @@ class Base(Configuration):
         "documentation_url": values.Value(
             None, environ_name="FRONTEND_DOCUMENTATION_URL", environ_prefix=None
         ),
+        "technical_documentation_url": values.Value(
+            None,
+            environ_name="FRONTEND_TECHNICAL_DOCUMENTATION_URL",
+            environ_prefix=None,
+        ),
         "external_home_url": values.Value(
             None, environ_name="FRONTEND_EXTERNAL_HOME_URL", environ_prefix=None
         ),