add CODEOWNERS, automatically manage waiting-for-author/review labels

[syphar]

This would automatically set some of the labels I currently always manually test, and often forget to set (but then again rely on them being set :) )

First of all it would add a CODEOWNERS file so a review gets requested from @rust-lang/docs-rs . I would understand if this it too much noise for the team and we could also limit the reviewers.

Also this would:

• set the "waiting for review" label when a review is requested
• set the "waiting for author" (and remove "review") when changes are requested (a review with just a comment wouldn't do this)
• remove both labels when the PR is approved.

[GuillaumeGomez]

This is super cool!

[syphar]

Based on the feedback I got I decided for a less intrusive way, and created a new team for reviewers: rust-lang/team#1107

When the team is merged & deployed, we can continue with this PR.

[syphar]

Hm... thinking about this a little longer, I'm thinking about actually dropping the labels and the logic.

The idea:

• "waiting for review" -> request review from docs-rs gh team
• "waiting for author" -> when a review was sent

Incorporating the draft-status this would mean the only useful status label (for me personally) would be "waiting for deploy".

Will revisit this when rust-lang/team#1107 is merged

[syphar]

I'll merge as-is for now, the new GH team was merged.

Potentially we could get rid of the labels and replace it with better searches.