Use "make mdlint" to check current status

Mostly related to whitespace (newlines / trailing),
lots of issues remaining to fix in the docs directory.

As usual linting takes most effort when you start,
but from there it is easier to keep code and docs clean.

* upping the Makefile version
* release notes for 0.31.0

release notes for 0.29.0, also bumps the ISO and the minikube binary version to 0.29.0