text: Title: Docs: Add information on the default OpenAPI files available
  Body: ### Summary
  
  In https://docs.litestar.dev/latest/usage/openapi we don't really *specifically* say anything about the availability of `$openapi_path/openapi.{json,yaml,yml}` except in 2 places, 1 of which says it is deprecated: 
  - https://docs.litestar.dev/latest/usage/openapi/ui_plugins.html#providing-a-subclass-of-openapicontroller (but just glossing over)
  - https://docs.litestar.dev/latest/usage/openapi/ui_plugins.html#configuring-the-openapi-root-path (but just glossing over)
  
  It would be nice to document this when people need them e.g., for Scalar API client's collection import feature