[rv_dm] Document the Debug System wrapper
[rv_dm] Document the Debug System wrapper

The debug system wrapper instantiates the PULP Debug System, which we
vendor in. The vendored-in code contains documentation that follows our
documentation standards.

This commit adds documentation for the debug system wrapper. It points
to the upstream documentation, and explains the additional functinality
provided by the wrapper.

Fixes #343

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan