doc: changes on some minor details

1. Add empty line to make image not too packed
2. Update armv8r64 yocto layer diagram
3. Consistent term on Armv8-R
4. Update EULA link
5. Add description to the entries in User Guide landing page

Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Change-Id: I6375216b68975ccd6a09f39b29231c1ef605ee8d
Issue-Id: SCM-4371