armv8r64-extras/zephyr: Upgrade to version 3.1.0

Also update documentation, including pinning links to the "latest"
documentation to 3.1.0

Issue-Id: SCM-4304
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I7f3032b0b4722270962b5da89b28111a3239520d