title: Path prefix for relative paths in Eleventy markdown links
body: <p>How can I prefix relative links defined in markdown documents when I run eleventy to build static pages from my markdown and templates.</p>
  <p>I use ./docs as the output directory to build the documentation pages from markdown documents and a html template in the ./eleventy directory of my project.</p>
  <p><code>npx eleventy --input ./eleventy --output ./docs --formats=md</code></p>
  <p>How could I prefix all the relative links with /foo ?</p>
  <p>I had a look at url filter and the pathprefix option for the build command but I don't get it.</p>
  <p>Where would I set the url filter? In the markdown? In the eleventy config?</p>