website/docs: add nginx-proxy-manager

Signed-off-by: Jens Langhammer <jens.langhammer@beryju.org>

website/docs/providers/proxy/_nginx_proxy_manager.md

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+For Nginx Proxy Manager you can use this snippet
+
+```
+# 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
+proxy_buffers 8 16k;
+proxy_buffer_size 32k;
+fastcgi_buffers 16 16k;
+fastcgi_buffer_size 32k;
+
+location / {
+    # Put your proxy_pass to your application here
+    proxy_pass          $forward_scheme://$server:$port;
+
+    # authentik-specific config
+    auth_request        /akprox/auth/nginx;
+    error_page          401 = @akprox_signin;
+
+    # translate headers from the outposts back to the actual upstream
+    auth_request_set $authentik_username $upstream_http_x_authentik_username;
+    auth_request_set $authentik_groups $upstream_http_x_authentik_groups;
+    auth_request_set $authentik_email $upstream_http_x_authentik_email;
+    auth_request_set $authentik_name $upstream_http_x_authentik_name;
+    auth_request_set $authentik_uid $upstream_http_x_authentik_uid;
+
+    proxy_set_header X-authentik-username $authentik_username;
+    proxy_set_header X-authentik-groups $authentik_groups;
+    proxy_set_header X-authentik-email $authentik_email;
+    proxy_set_header X-authentik-name $authentik_name;
+    proxy_set_header X-authentik-uid $authentik_uid;
+}
+
+# all requests to /akprox must be accessible without authentication
+location /akprox {
+    proxy_pass          http://*ip or hostname of the authentik OUTPOST*:9000/akprox;
+    # ensure the host of this vserver matches your external URL you've configured
+    # in authentik
+    proxy_set_header    Host $host;
+    add_header          Set-Cookie $auth_cookie;
+    auth_request_set    $auth_cookie $upstream_http_set_cookie;
+}
+
+# Special location for when the /auth endpoint returns a 401,
+# redirect to the /start URL which initiates SSO
+location @akprox_signin {
+    internal;
+    add_header Set-Cookie $auth_cookie;
+    return 302 /akprox/start?rd=$request_uri;
+}
+```