add styles for TOC and integrate TOC into page layout

add styles for TOC and integrate TOC into page layout

- use flexbox to layout TOC
- add scroll spy behavior to sidebar TOC
- introduce additional breakpoint for sidebar TOC width (desktop and widescreen)
- activate last entry in toc if scroll has reached bottom of page
- introduce variables for toc (top, width, etc)
- add utility class to hide element when in TOC
- look for -toc class on body; rename before-toc class to is-before-toc
- use article top as threshold for activating entry in TOC
- disable scrollbar on toc in Firefox
- scroll list instead of menu
- hide scrollbar on TOC
- don't add embedded toc if element is found with ID "toc"