document the need for Mapbox access token if maps are required on UI

[konstan]

Example:

Mapbox access token

For the Portal to show the maps, it needs to connect to the https://www.mapbox.com/ API. For that, it needs an 
API access token. You need to register with the service and get the access token from https://account.mapbox.com/. 

The access token must be set in the ui-config.json under mapbox-access-token key. Copy the 
ui-config.json.example file to ui-config.json and set the proper value for the key.

Implement providing ui-config.json and document according to the implementation.