Label cleanup for docs repo

[roji]

We've been cleaning up our labels in the main EF repo, and converting them to "Closed" reasons where possible, but we've left the docs repo untouched.

Here's a proposal:

• For closed-question, closed-external, closed-by-design, closed-wont-fix, closed-cannot-reproduce, closed-no-response and closed-not-needed, we should just close the issue with the reason "not planned" (unless it already is), and then kill the labels.
  • Note that it's better to just delete the label itself once at the UI at the very end - this easily/silently removes the label from all issues without posting a useless message.
• For closed-fixed, make sure the the issue is closed with the reason "fixed", and then kill the label.
• This leaves just closed-duplicate, where we're in the same situation as in the product repo - the Github API doesn't yet allow scripting that, so we can leave it for now.