Configure CODEOWNERS for datawire/ambassador-docs

[jastang]

Why?
Documentation PRs are an easy entrypoint for community contributions. However, members don't have the ability to tag reviewers, nor do they know who the best person to tag is. PRs might stay open and go stale as a result.

What?
Per a quick Slack thread with @kflynn, a CODEOWNERS configuration is an easy solution.

[jastang]

I'd also be happy to take this once we know all the team gh handles.