The files in this directory are included in the course via the {{#include ..}}
syntax. All third-party content must be placed here to clearly indicate its
origin.
When we publish a translation of the course, we git restore the src/ and
third_party/ directories at the repository root back to the date listed in the
POT-Creation-Date header of the translation. It is crucial, that all
translatable content lives in those two directories. The other files (such as
book.toml and theme/) are not restored and we always use the latest version
of them.