Improve usability of documentation

[nimo23]

1. Please provide a fixed left sidebar for TOC (Table of Contents). Currently, the TOC scrolls with the content which is uncomfortable. And please use toggles for the main concepts of the TOC. Currently, the user must scroll a lot up and down. For inspiration, look at

• https://lodash.com/docs/4.17.15 (fixed left sidebar with collapsable items for TOC)
• https://reactjs.org/docs/getting-started.html (fixed right sidebar with collapsable items for TOC)
• https://developer.microsoft.com/en-us/fluentui#/controls/web/button (fixed left sidebar with collapsable items for TOC)
• https://flutter.dev/docs (fixed left sidebar with collapsable items for TOC)

2. And please consider to use another more readable font for better readability. At least, make the font-size and line-height a little smaller. Currently, the font-size (for desktop) is 16 (and line height is 1.6) which is, in my opinion, very uncomfortable to read. Compare the font (size and line-height) with the others example above (which uses, for example, font-size: 14, line-height: 1.3).