tl; dr: Use
<<Markdown(some_url)>> to render Markdown on wiki.ros.org
A lot about open source has changed since ROS started. Notably, a lot of maintainers prefer to put their documentation on Github READMEs rather than an independent wiki like wiki.ros.org.
As of this PR you can now have the ROS Wiki render your remotely-hosted Markdown content.
For example, the above snippet will fetch the given URL and render it onto the page.