Add action for wiki change notifications.

[jonmeow]

Wiki can either be "require push access" or "everyone" -- apparently there's no other option. I've set it to "everyone" so that contributors can make edits without needing push access. So the options as I see it are:

• Leave wiki as "everyone" can edit, use this for notifications.
  • GitHub doesn't give notifications for wiki edits. This is trying a different approach for notifications.
• Grant push access to a larger group (contributors), don't add CODEOWNERS.
  • Not sure this is the right choice because of the implications around merges, but again maybe it'd be fine and we can expect the approval requirement to work out.
• Grant push access to contributors, add CODEOWNERS.
  • This causes the auto-assignment to CODEOWNERS that we don't want. (details in Remove CODEOWNERS #1367)
• Create a separate wiki repo so that we can differently handle push access.
  • This seems overly complex a solution though.

I'm hoping this approach works.

(note, I haven't set up the webook yet, wanted agreement on trying the approach first)

[chandlerc]

I'm happy starting with this approach. We can see how it goes.