QDoc: Move Generator::typeString into NodeText::

`Generator::typeString()` is a virtual method on Generator that isn't
overridden anywhere and that doesn't rely on Generator's state. Thus, it
shouldn't be `virtual`; also, it should be either static or live
somewhere else.

In preparation for a change that will make use of the function from
`NodeExtractor`, move the function into a new namespace,
`NodeText`. Update all call sites.

Pick-to: 6.12
Task-number: QTBUG-151328
Change-Id: I6cc75d6e678a44c42f60ed1727b790481faaa07b
Reviewed-by: David Boddie <david.boddie@qt.io>
Reviewed-by: Topi Reinio <topi.reinio@qt.io>