docs: add ci configuration page
docs: add ci configuration page

This page contains initial information on Rook's CI setup, such as
environment variables.

Signed-off-by: Alexander Trost <88b6b84d9fa0dbf878e34ae3d402bc4895fcd37e@googlemail.com>

rook/rook,rook/rook,rook/rook,rook/rook,rook/rook