Closes #50 (closed)
This MR provides the following adjustments to the theme.
- Allow for heading level 6 in the ToC.
- place the toc in a sticky aside
- this is not a perfect solution, as a long toc will runs out of the screen
- set z-index on centered avatar image to place the top menu in front
- to be readable in front of the avatar (see ↑) the top menu sets a background-color same as the header behind