Adds a basic document giving an introduction to `cheriot-audit`, a short
guide to setting it up, a brief introduction to using it, and pointers
to appropriate upstream sources of documentation.

This also includes a quick primer on Rego, with an introduction to some
of the language's features and syntax.

This commit introduces exercise 1 for using CHERIoT Audit to audit
firmware alongside Rego policies. This exercise involves writing a
policy to detect if firmware uses sealed capabilities, and to query
information about these capabilities if they exist.

This commit introduces exercise 2 for using CHERIoT Audit to audit
firmware alongside Rego policies. This exercise involves writing
a policy to ensure that only specified functions are running
with interrupts disabled.

This commit introduces exercise 3 for using CHERIoT Audit to audit
firmware alongside Rego policies. This exercise involves writing
a policy to ensure that specific allocation limits are respected, with
individual allocation limits per allocation capability, per compartment
(total) and across all compartments.