doc: Improve the instructions for the final phase of distro installation

Major changes:

* For Debian installation, add steps after doing the GRUB workaround

* Add descriptions for finishing the installation and rebooting the
  installed Debian

Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>