3.1: Add a line/paragraph on scanner

[habibasorour]

Description

Adding a small paragraph/line to elaborate on some of the scanner methods used, like .nextDouble(), and what a scanner variable is.

Screenshot

[habibasorour]

found in the old rst a good paragraph about it. It should be re-added.
https://runestone.academy/ns/books/published/java4python/Java4Python/javadatatypes.html?mode=browsing#input-output-scanner

With the addition of this paragraph, section 3.1 would need a bit more restructuring.

[pearcej]

This book is significantly changed from the first version, but perhaps it makes sense to do a pass through the rst version looking for missing content.