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

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

website/docs/providers/proxy/_nginx_standalone.md

@@ -1,4 +1,10 @@
 ```
+# Upgrade WebSocket if requested, otherwise use keepalive
+map $http_upgrade $connection_upgrade_keepalive {
+    default upgrade;
+    ''      '';
+}
+
 server {
     # SSL and VHost configuration
     listen                  443 ssl http2;
@@ -18,6 +24,9 @@ server {
         # proxy_pass http://localhost:5000;
         # 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