Add slide on `std::numbers`

[bernhardmgruber]

In C++20 we got the new header <numbers>, which among other things defines std::numbers::pi. This replaces the common use of unportable constants like M_PI.

[stale[bot]]

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.