Sticky TOC for Developer Handbook

[kahboom]

The Developer Handbook/Docs can be quite long at times. If we are not planning to separate into smaller chunks of content, we should make the table of contents "sticky," so that when the user scrolls they can still have access to it.

The same goes for a "Top" button at the bottom, that would push users back to the top of the page when they press it.

[kahboom]

cc @zregvart