Fix Table of Contents

[RicCu]

When viewing the docs on a narrow browser window, the table of contents is collapsed and placed at the top of the page.

However, this table is lost when you scroll down, forcing you to scroll back to the top of the page to navigate somewhere else.

It would be nice to have it fixed to the top or have it otherwise easily accessible at any point on the page (maybe collapse it as you scroll down and have it pop up again as you scroll up?).