Request: Add unified documentation for slots / slot property

[aprixon01112017]

I was looking for documentation about slots usable with tables.No luck.
I propose a dedicated paragraph about usable slots on each and every relevant component doc page.

[rossity]

Agreed. Apparently there are slots on the QField component (super useful to my use case) and they were not in docs, had to look through the component source.

[rstoenescu]

This is part of docs website 2.0 rewrite. Will be available soon.