Control which page to show when the root index / is accessed.
Same as in legacy, with semantic configuration, by siteaccess.

• This should be done with a kernel.request event listener.
• RequestEventListener class can be re-used.
• Priority of such listener must be greater than onKernelRequestForward (10), but lower than the router (32). Ideal value might be 15

Technically, the following should happen:

• If the semanticPathinfo request attribute equals to / AND the request is a master request, we retrieve the indexPage setting through the ConfigResolver.
• If the indexPage parameter is not null (null should be the default value):
  • Replace semanticPathinfo request attribute by this value.
  • Set needsForward request attribute to true.