Frontend configuration instructions and adding config for the default URI

[ZakarFin]

• Make default URI configurable for fetching data from the rest API with conf.js registryApp.defaultRegisterURI. Currently the frontend tries to query the registry for content to show with URL where the frontend is running in by default. If the URI in the database doesn't match the frontend location, this new conf can be used to make the frontend query the correct URI for the default listing.
• Add comments where paths inconf.js should point to/be redirected to on httpd.

[ZakarFin]

These enable the browsing the data for the frontend: https://github.com/ec-jrc/re3gistry/blob/v2.5.2/sources/Re3gistry2ServiceWebapp/support_files/re3gistry2-webapp.conf#L39-L111

However you need handle every new path you want to use in the frontend app in these: https://github.com/ec-jrc/re3gistry/blob/v2.5.2/sources/Re3gistry2ServiceWebapp/support_files/re3gistry2-webapp.conf#L93-L106