New landing page for stb-tester.com

The existing developer documentation needs to be migrated to the new
markdown template at some point. I originally used reStructuredText for
compatibility with the Python ecosystem, sphinx, etc, but stb-tester
will probably always have enough non-python bits (shell, C) that it will
never be a full python solution so we won't be able to package it on
pypi. And I do dislike reStructuredText.

Building the new html requires pandoc, which you'll find in your package
manager.