doc.qt.io: Improve usability of table of contents

The recently-updated online documentation page makes it easy to navigate between supported Qt versions, which is great. However, the Table of Contents (ToC) has become harder to use.

• At 100% scaling, the ToC disappears completely if the browser window is <= 1440px wide. On a regular 1080p screen, users who don't keep their browser window maximized might not even realize that a ToC exists.
• The ToC does not scroll until you reach the very bottom of the page. This means the last few links of a long ToC might be hard to access.