Update getting_started.rst

noob-proofing a bit
Fix Buffalo WZR-HP-AG300H/WZR-600DHP image name
Simplify git commands in Getting Started
Merge branch 'master' of https://github.com/s72785/gluon into s72785-master
Merge branch 'master' of https://github.com/FreifunkBremen/gluon into FreifunkBremen-master

Conflicts:
	docs/user/getting_started.rst
Merge branch 'FreifunkBremen-master'
docs: advise user to checkout a specific release