Refine platforms guide
Refine platforms guide

This gives the platforms guide a bit more love and fixes up some nits
which came across my eye. The overall content will not change.

Signed-off-by: Sascha Grunert <1d6e1cf70ec6f9ab28d3ea4b27a49a77654d370e@saschagrunert.de>

kubernetes/sig-release