Library Docs: Redesign Embedded TOC

[julioest]

Description

On screens smaller than 1024px, the TOC is currently embedded between the heading and the main content (see screenshot). This placement can potentially negatively impact the user experience, especially with a long list.

Some initial feedback from Vinnie:

Previously, this was a nifty control to the right. I prefer that, if it is going to be visible. However, one of the options is to disable that control, which I prefer even more over having it at all (for URL). If that option disables the control it should not be visible at the top of the page.

Ref: boostorg/website-v2#1200 (comment)