starlight_help: Forward to help_center_port in case of dev server.

Fixes
https://chat.zulip.org/#narrow/channel/19-documentation/topic/new.20help.20center.3A.20run-dev.20options/near/2238128.

When running the dev server, astro/starlight makes a lot of requests on
the base URL, which are not scoped to /starlight_help. We need to host
the dev server on it's own port for it to work well. This commit fixes
that.

templates/zerver/development/dev_tools.html

@@ -173,6 +173,14 @@
                 want to visualize the changes quickly in the beta help center documentation. You will need the
                 converted MDX files to be already generated through
                 <code>./tools/convert-help-center-docs-to-mdx</code> before you run the dev server with this flag.
+
+                <ul>
+                    <li>
+                        The dev server makes a bunch of request to base Zulip URL instead of scoping it to the astro/starlight
+                        base url. For this reason, we run the dev server on it's own port and redirect help center requests to
+                        the appropriate port (9995 by default).
+                    </li>
+                </ul>
             </li>
         </ul>
     </div>

tools/run-dev

@@ -345,11 +345,13 @@ The instructions for the build step can be found in the `./devtools`
 page. `/starlight_help` urls will give you an error until you complete the
 build step and rerun `run-dev`.""")
     elif enable_help_center_dev_server:
+
+        async def redirect_help(request: web.Request) -> web.HTTPFound:
+            return web.HTTPFound(request.url.with_port(help_center_port))
+
         app.add_routes(
             [
-                web.route(
-                    hdrs.METH_ANY, r"/{path:(starlight_help).*}", partial(forward, help_center_port)
-                ),
+                web.route(hdrs.METH_ANY, r"/{path:(starlight_help).*}", redirect_help),
             ]
         )
     app.add_routes(