Architecture docs: interface-overview

[Joseph Reynolds]

I created an overview of the BMC's primary external interfaces and 
selected internal interfaces.  We need these to talk about which 
interfaces we want to let BMC admins enable and disable.  These also 
form the attack surfaces of the BMC, so are interesting for security 
analysis.

https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/27969

It's still a bit of a work-in-progress, but I think it can be merged 
after a little clean up.  My idea is for project built on OpenBMC to 
start with this base, and extend it in various ways, for example, to the 
AST2500 or to the OpenPOWER host.

Please take a look.

- Joseph