Portal - add documentation on categories

[mouligno]

We should add some documentation on the default portal categories :

• Starred / Les mieux notées - based on ratings, APIs with ratings, sort by average rate then rating count then alphabetical order
• Trending / Les plus tendances - based on accepted and paused subscriptions count, sort decreasing
• Featured / A la une - top apis are configured at environment level

Maybe addition on this page https://docs.gravitee.io/apim/1.x/apim_consumerguide_subscribe.html or create a global portal presentation page ?