Documentation: Update and extend interface descriptions

First of all, introduce a glossary for key terms used in the Jailhouse
docs. It introduces the new terms "root cell" and "non-root cell". Then
extend and refactor the hypervisor interface descriptions, specifically
adding hypercalls for obtaining some basic diagnostic data. This data is
supposed to be exposed by the driver via sysfs. Start documenting its
structure.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>