website/docs: add websocket support to nginx snippets (#9220)

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

website/docs/providers/proxy/_nginx_proxy_manager.md

@@ -1,6 +1,10 @@
-For Nginx Proxy Manager you can use this snippet
-
 ```
+# Upgrade WebSocket if requested, otherwise use keepalive
+map $http_upgrade $connection_upgrade_keepalive {
+    default upgrade;
+    ''      '';
+}
+
 # Increase buffer size for large headers
 # This is needed only if you get 'upstream sent too big header while reading response
 # header from upstream' error when trying to access an application protected by goauthentik
@@ -16,6 +20,9 @@ location / {
     # Set any other headers your application might need
     # proxy_set_header Host $host;
     # proxy_set_header ...
+    # Support for websocket
+    proxy_set_header Upgrade $http_upgrade;
+    proxy_set_header Connection $connection_upgrade_keepalive;
 
     ##############################
     # authentik-specific config