Add HAProxy delegation example to docs (#12501)

Signed-off-by: Ville Petteri Huh

Author: villepeh

changelog.d/12501.doc

@@ -0,0 +1 @@
+Add HAProxy delegation example with CORS headers to docs.

docs/reverse_proxy.md

@@ -206,6 +206,28 @@ backend matrix
   server matrix 127.0.0.1:8008
 ```
 
+
+[Delegation](delegate.md) example:
+```
+frontend https
+  acl matrix-well-known-client-path path /.well-known/matrix/client
+  acl matrix-well-known-server-path path /.well-known/matrix/server
+  use_backend matrix-well-known-client if matrix-well-known-client-path
+  use_backend matrix-well-known-server if matrix-well-known-server-path
+ 
+backend matrix-well-known-client
+  http-after-response set-header Access-Control-Allow-Origin "*"
+  http-after-response set-header Access-Control-Allow-Methods "GET, POST, PUT, DELETE, OPTIONS"
+  http-after-response set-header Access-Control-Allow-Headers "Origin, X-Requested-With, Content-Type, Accept, Authorization"
+  http-request return status 200 content-type application/json string '{"m.homeserver":{"base_url":"https://matrix.example.com"},"m.identity_server":{"base_url":"https://identity.example.com"}}'
+
+backend matrix-well-known-server
+  http-after-response set-header Access-Control-Allow-Origin "*"
+  http-after-response set-header Access-Control-Allow-Methods "GET, POST, PUT, DELETE, OPTIONS"
+  http-after-response set-header Access-Control-Allow-Headers "Origin, X-Requested-With, Content-Type, Accept, Authorization"
+  http-request return status 200 content-type application/json string '{"m.server":"matrix.example.com:443"}'
+```
+
 ### Relayd
 
 ```