numbers concept: improve about.md and introduction.md

I think we can introduce the [numbers concept](https://github.com/exercism/go/tree/main/concepts/numbers) a little bit better. Right now, the `about.md` and `introduction.md` file provide basic info about numbers that I think could be improved with the following:

* Add more examples in general. Give more examples of integer variable declarations, arithmetic operators being used, type conversions and their result, etc...
* Mention alternative notations to declare numbers: the possibility to add `_` between digits, the scientific notation, binary and hexadecimal literals.
* Mention briefly complex numbers, with 1 or 2 examples.

Maybe leave point 2 and 3 only for `about.md` so students are not overwhelmed in `introduction.md`.