The current user guide mentions that Foundation model doesn't support
debugger interface. Clarify that all FVPs support --cadi-server option
such that a CADI-compliant debugger can connect to and control model
execution.

Also fix broken URL to FVP home page.

Change-Id: Ia14d618a4e0abb4b228eb1616040f9b51fb3f6f9
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>