integrations: Add docs for GitHub Actions integration.

I have added a documentation page for the GitHub Actions integration to
`/integrations/doc/github-actions` with a link to the Zulip GitHub
Actions repository.

Tweaked by tabbott to add cross-links with the main GitHub integration.

templates/zerver/integrations/github-actions.md

@@ -0,0 +1,8 @@
+It's easy to send Zulip messages from GitHub Actions! This is useful:
+
+  * If you want to report the success or failure of an Action to Zulip.
+  * If you want to send output from your actions to Zulip.
+
+Instructions are available [here](https://github.com/zulip/github-actions-zulip#readme).
+
+See also the [GitHub integration](/integrations/doc/github).

zerver/lib/integrations.py

@@ -489,6 +489,13 @@ INTEGRATIONS: Dict[str, Integration] = {
     "git": Integration(
         "git", "git", ["version-control"], stream_name="commits", doc="zerver/integrations/git.md"
     ),
+    "github-actions": Integration(
+        "github-actions",
+        "github-actions",
+        ["continuous-integration"],
+        display_name="GitHub Actions",
+        doc="zerver/integrations/github-actions.md",
+    ),
     "google-calendar": Integration(
         "google-calendar",
         "google-calendar",

zerver/webhooks/github/doc.md

@@ -17,3 +17,5 @@ Get GitHub notifications in Zulip!
 {!congrats.md!}
 
 ![](/static/images/integrations/github/001.png)
+
+See also the [GitHub Actions integration](/integrations/doc/github-actions).