arm-bsp/docs: documentation improvements for fvp-baser-aemv8r64

This patch updates the documentation for fvp-baser-aemv8r64 :

- Update link to the yocto documentation.
- Add details about what Host OS version are supported. Add Ubuntu
20.04 specific instructions.
- Warn user about disk space.

Issue-Id: SCM-2665
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Change-Id: Ie93a42c2a1ec595707fa7496edbf024650f7eb85
Signed-off-by: Jon Mason <jon.mason@arm.com>