refs #18778 [fid_bbi] static pages

* add navigation links to static pages
* add new about page
** In German and English.
* add new FAQ page
** In German and English.
** Including style fixes for accordions and nested lists with paragraphs.
* add new "Für Fachreferate" page
** This page is only available in German.
* improve heading dimensions
** Increase 1st level heading size on small screens and limit its width.
** Reduce margin between 1st and 2nd level headings.
* add legal notes ("Impressum")
** In German and English,
** and update footer link.
* update static pages
** Extend FAQ
** Rename "subject specialists" to "subject librarians"
** Change "bibliography" to "book studied"