`constexpr` should be mentioned and discussed on the `const` page

`constexpr` is preferred over `const` for C++ compile-time constants, and should be mentioned and discussed here:

https://www.arduino.cc/reference/en/language/variables/variable-scope--qualifiers/const/

Here's some of the differences to be aware of:
https://stackoverflow.com/questions/13346879/const-vs-constexpr-on-variables/13347355#13347355

Start with the Arduino Reference const page linked in the issue, then read the linked comparison of const and constexpr. Update that page to mention and discuss constexpr for C++ compile-time constants, with the relevant differences covered; completion is a clear, published documentation change on that page.

Written by the indexing model from the issue text.