Add sampling profiler visualisation

Add an interactive JS visualisation to demonstrate how the sampling
profiler works. A small demo program is shown executing line by line,
demonstrating how stack frames are created and destroyed and how samples
are taken periodically during the process.

A new Sphinx extension is used to capture and inject the trace into the
JS file during documentation build to make these traces reproducible
(and avoid hard coding them in the source code).

Author: lkollar