Implement GitHub source code links in docs app & generation script

[Eric-Arellano]

This is the final part of #454. It's blocked by #518 and #517.

Once we have a design, we need to implement it. We'll need to decide on what CSS class names to use etc and make sure that they're preserved by our API generation script. This will relate to the overall improvement of API docs, where we also are going to need to start preserving class names so that we can have specific styling rules: #479.

[Eric-Arellano]

Closed by #620. We'll need to add an additional implementation for the linkcode extension as part of #517, but that is tracked by #517.