Integrate doc subdir

[garrett]

The doc subdir has several Markdown files that could be integrated and searchable from the website:
https://github.com/cockpit-project/cockpit/tree/master/doc

[garrett]

@martinpitt: Should we sync in some of these .md documents as external docs? If so, which?

[martinpitt]

Most of them are really developer docs which you'd only want to look at when actually working on the cockpit source code. authentication.md and protocol.md are interesting in general for third-party developers. We already link to authentication.md from https://cockpit-project.org/guide/latest/authentication.html for example. At the moment I'm not sure where they would fit on our homepage, though. Perhaps they could be linked from https://github.com/cockpit-project/cockpit/wiki/Contributing ?