Documentation: Rewrite protocol specification

The rewrite addresses two main issues in the original documentation:

1. The conflation of the protocol specification with the mailbox transport
2. Formatting and discoverability of command and event definitions

Additionally, the rewrite documents two new transports - IPMI and DBus.
It's noted that DBus is intended as a transport internal to the BMC,
while the IPMI transport is the new transport exposed to the host.

Finally, some commands and events have been renamed, however this has no
impact on the behaviour of the protocol.

Change-Id: Icc78141f4ead4395e8a348b80443cadd2300a751
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>