MLECO-3010 Minor clarifications in quick start guide

Tried to clarify location of axf file and need to change directory at a specific step
Added note on changing permissions to AVH guide and added link to AVH guide in
quickstart.md as well as recommending a VNC viewer if using AVH

Change-Id: I653f0ac3ae52a6fc1c44e28024b9513af89c6db6
Signed-off-by: Liam Barry <liam.barry@arm.com>