Surface "orphaned" content

[jugglinmike]

The following files are not referenced from any of WPT's existing documentation:

• docs/_writing-tests/testdriver.md
• docs/_writing-tests/testdriver-tutorial.md
• docs/_reviewing-tests/email.md
• server-side.md
• css/README.md
• resources/readme.md
• tools/wptrunner/README.rst

This all useful information, but readers are unlikely to find it in its current location. If it isn't already located in WPT's docs/ directory, it should be moved there so that it can be included in http://web-platform-tests.org. In some cases, maintainers may prefer to persist the content within its current location. In any case, we should update the "main" documentation with references to all of this content.

[Simon Pieters]

I think we shouldn't move files at this time, since we might decide to switch the build setup such that arbitrary files can be included. As a starting point, any docs that is in the repo but isn't in docs/ could be linked to the rendered version on GitHub. Then we can revisit this issue after we have decided one way or the other on changing the build setup.