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

[ElectricRCAircraftGuy]

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