Documentation improvements

[gadenbuie]

• How to add custom CSS (see Modifications to Learnr User-Interface #716 (comment))
• progressive and allow_skip should be discussed in Interactive Questions

[luk-brue]

maybe also

• how to make sure css is found when deploying on a server (had this problem)
• Publishing Basics: add link to https://r-pkgs.org/ for learning how to create package
• maybe also a section about commonly used css styles: for me, it was all about infoboxes.
• also regarding css styling: add link to this helpful section about div tags and the easier ::: implementation
  https://pandoc.org/MANUAL.html#divs-and-spans
• add this resource about tabs: https://bookdown.org/yihui/rmarkdown-cookbook/html-tabs.html