doc: Clarify the current System Requirements

The Kronos Reference Stack has only currently been tested directly in
a non-headless desktop environment so clarify the System Requirements
to this effect and add a note to the Limitations in the Changelog.

Increase the free disk space requirement to 300GiB to account for Linux
Distribution Installation.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>