gh-70647: Better promote how to safely parse yearless dates in datetime. (GH-116179)

* gh-70647: Better promote how to safely parse yearless dates in datetime.

Every four years people encounter this because it just isn't obvious.
This moves the footnote up to a note with a code example.

We'd love to change the default year value for datetime but doing
that could have other consequences for existing code.  This documented
workaround *always* works.

* doctest code within note is bad, dedent.

* Update to match the error message.

* remove no longer referenced footnote

* ignore the warning in the doctest

* use Petr's suggestion for the docs to hide the warning processing

* cover date.strptime (3.14) as well