docs: add some info on alternate install methods

"Alternate" here means "non Live/Install Image".

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: alternate install method for Debian Wheezy with Preempt-rt

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: add a "Download the image" section

This moves "Normal Download" and "Multi-session Download" down one level
in the section hierarchy.

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>
docs: add a "Download using zsync" section

Signed-off-by: Sebastian Kuzminsky <seb@highlab.com>