Pod-Html: document new location for utility functions

Pod-Html: document new location for utility functions

... along with guidance on changes in perl-5.38.

Pod::Html::Util: caution on use of these subroutines outside core.  For
perl-5.36, these three utility functions will still be importable from
Pod::Html, but thereafter they will only be importable from
Pod::Html::Util.  (They are simply imported and re-exported per
suggestion from Graham Knop in GH 19036.

Also, (i) add explicit tests for anchorify() and relativize_url() based
on how they are used in 'installhtml'; (ii) conduct these tests imported
from both Pod::Html::Util (permanently) and Pod::Html (during perl-5.36
only).

For: https://github.com/Perl/perl5/pull/19036