numbers concept: improve about.md and introduction.md

I think we can introduce the numbers concept 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.