Add a PDF document link on the dev server homepage which uses a URL that
has a random suffix parameter added on page load and each time the link
is clicked.

This is useful for testing the behavior of client features (eg.
annotation fetch and real time updates) which should show the same
content/notifications etc. across copies of the "same" document
presented at different URLs.

This will aid with testing changes such as https://github.com/hypothesis/h/pull/6542.