chore: Improve Helpcenter custom domains (#5456)

- Support rendering articles over frontend URL
- Support rendering articles over help center URL
- Support rendering help center home page in the custom domain root

.env.example

@@ -3,6 +3,8 @@ SECRET_KEY_BASE=replace_with_lengthy_secure_hex
 
 # Replace with the URL you are planning to use for your app
 FRONTEND_URL=http://0.0.0.0:3000
+# To use a dedicated URL for help center pages
+# HELPCENTER_URL=http://0.0.0.0:3000
 
 # If the variable is set, all non-authenticated pages would fallback to the default locale.
 # Whenever a new account is created, the default language will be DEFAULT_LOCALE instead of en