Add action for wiki change notifications. (#2089)

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 #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.

.github/workflows/discord_wiki.yaml

@@ -0,0 +1,15 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: Wiki Changed Discord Notification
+
+on: gollum
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: 'oznu/gh-wiki-edit-discord-notification@v1.0.0'
+        with:
+          discord-webhook-url: ${{ secrets.DISCORD_WEBHOOK_WIKI_EDIT }}