Sync web-platform-tests PR 19647 into mozilla-central (this bug is closed when the sync is complete).
Details from upstream follow.
Mike Pennisi <firstname.lastname@example.org> wrote:
[docs] Include wptrunner docs on web-platform-tests.org
The wptrunner module has a substantial amount of documentation describing its usage and internals. It was authored to be built as a standalone web site using a prior release of the Sphinx documentation generator.
Update the structure of the existing wptrunner documentation so that it may be built in the context of the WPT documentation website. Extend the existing documentation with two references to the new content: one in the "Tooling" section of the "Project Administration" page, and one in the instructions on running the tests from the command line. Remove the code used to generate a standalone website for wptrunner.