help: Do not allow autolinks for help center docs.

mdxjs does not allow autolinks.
See mdx-js/mdx#1049.
We also added a linter check for the same.
Preparatory commit for zulip#30451.

help/emoji-and-emoticons.md

@@ -132,7 +132,7 @@ names.
 
 !!! tip ""
 
-    <https://emojipedia.org/> may be a helpful resource.
+    [https://emojipedia.org/](https://emojipedia.org/) may be a helpful resource.
 
 {end_tabs}
 

help/include/legacy-log-in.md

@@ -1,4 +1,4 @@
-1. Go to <https://selfhosting.zulip.com/serverlogin/>.
+1. Go to [https://selfhosting.zulip.com/serverlogin/](https://selfhosting.zulip.com/serverlogin/).
 
 1. Fill out the requested server information, and click **Continue**.
 

help/linking-to-zulip-website.md

@@ -15,7 +15,7 @@ website (e.g., **Sponsors** or **Acknowledgements**).
    logo](https://github.com/zulip/zulip/tree/main/static/images/logo),
    preferably a **round Zulip icon** (available in `.png` and `.svg` formats).
 
-1. Link to <https://zulip.com/>.
+1. Link to [https://zulip.com/](https://zulip.com/).
 
 2. If appropriate, add a brief description:
 

tools/linter_lib/custom_check.py

@@ -839,6 +839,14 @@
         {
             "pattern": r"\[(?P<url>[^\]]+)\]\((?P=url)\)",
             "description": "Linkified Markdown URLs should use cleaner <http://example.com> syntax.",
+            "exclude": {"help/"},
+        },
+        {
+            "pattern": r"<http(s?)://[^>]+>",
+            "description": """Autolinks are not allowed in /help documentation due to the upcoming migration to mdx.
+            Use Linkified markdown URLs [url](url) instead.
+            See https://github.com/mdx-js/mdx/issues/1049 for more info.""",
+            "include_only": {"help/"},
         },
         {
             "pattern": "https://zulip.readthedocs.io/en/latest/[a-zA-Z0-9]",