Label cleanup for docs repo

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.