Add mechanism to easily access the sidebar TOC menu

[hamishwillee]

For some large topics, like Getting Started the navigation scrolls out of view. If you want to access it then you have to do a lot of scrolling up to get back to it.

The requirement is to create a mechanism to make it easy to get to the sidebar TOC again. Some ideas:

• A "jump to top" button that is visible when the sidebar toc is off-screen and you are scrolling up
• The sidebar will only scroll as far as the bottom of its content, irrespective of what the main content does (always on-screen). Scrolling up in content also scrolls up the sidebar.
• Other?