Some work on py-nutshell.rakudoc

[schultzdavid]

This will partly address issue #2354.

Note to self:

Add a rudimentary section on Raku's type system below the section on variables. In this new section, include among other things the distinction between definite and indefinite objects.

In the section "Control flow", include (a little bit) of material on given and // – mostly to show what would be Raku equivalents of the common Python idiom if x is not None. Perhaps split the control flow section into the two subsections "Conditionals" (if, given, when...) and "Iteration" (while, for, loop...).