Mention `std::complex`

[bernhardmgruber]

A participant in the spring course asked about std::complex. I think we should at least mention it's existence.

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.