integrations: Add documentation for Mastodon via public RSS feeds.

This uses the Mastodon logo SVG as pulled from
https://blog.joinmastodon.org/logo.svg.

templates/zerver/integrations/mastodon.md

@@ -0,0 +1,35 @@
+Fetch public posts (sometimes called “toots”) of individual accounts or
+federated hashtags from Mastodon in Zulip! While Zulip doesn't integrate
+directly with ActivityPub or the overall Fediverse, some ActivityPub servers,
+like [Mastodon](https://joinmastodon.org/), publish RSS feeds that can
+be followed in Zulip using the [Zapier][1] or [RSS][2] integrations.
+
+!!! warn ""
+
+    Due to the complexities of Mastodon federation,
+    following a hashtag on one homeserver does not guarantee that all posts in the
+    entire Fediverse will be included in the feed.  Rather, all public posts
+    that have federated to the instance in question will be included. This
+    means you may get different results for the same hashtag, depending on
+    which homeserver you choose to subscribe through.
+
+1. Find the RSS feed for the account or hashtag you'd like to follow. Usually,
+   this means appending `.rss` to its Mastodon URL.
+
+    !!! tip ""
+
+        For example, to follow Zulip's Mastodon account at
+        `https://fosstodon.org/@zulip`, you would use
+        `https://fosstodon.org/@zulip.rss`. To follow the **#zulip** hashtag at
+        `https://fosstodon.org/tags/zulip`, you would use
+        `https://fosstodon.org/tags/zulip.rss`.
+
+1. Follow the [Zapier][1] integration guide (recommended) or the [plain RSS][2]
+   integration guide using this feed URL.
+
+{!congrats.md!}
+
+![Mastodon posts in Zulip via Zapier](/static/images/integrations/mastodon/001.png)
+
+[1]: ./zapier
+[2]: ./rss

zerver/lib/integrations.py

@@ -562,6 +562,13 @@ INTEGRATIONS: Dict[str, Integration] = {
         display_name="Jitsi Meet",
         doc="zerver/integrations/jitsi.md",
     ),
+    "mastodon": Integration(
+        "mastodon",
+        "mastodon",
+        ["communication"],
+        display_name="Mastodon",
+        doc="zerver/integrations/mastodon.md",
+    ),
     "matrix": Integration(
         "matrix", "matrix", ["communication"], doc="zerver/integrations/matrix.md"
     ),