Write a man-page

[connorkuehl]

One can interrogate sevctl and its subcommands with increasingly long sevctl blah --help lines, but it'd be cool if all of this was documented in a man page.

[berrange]

The --help output is pretty terse for the commands, so not really sufficient unless the users are already very familiar with SEV concepts. A man page is definitely required here to get better overall usage guidance, and especially outline expected workflow for utilizing the various commands.