Factor out mainpage features into dedicated pages

[tisonkun]

Currently, our main page contains three featured topics:

This is good while the details can be too many to follow.

I'd suggest we keep "Languages", "Services", and "Layers" title and the one line brief, but move the outline points into docs pages so that we keep the main page concise while users can dive into details with a hook.

cc @Xuanwo @suyanhanx