www/pmwiki: Update 2.5.4 => 2.5.8, take maintainership

Changelogs:
https://www.pmwiki.org/wiki/PmWiki/ChangeLog#v259

Release Notes:
https://www.pmwiki.org/wiki/PmWiki/ReleaseNotes#v258

Improve port:

• Replace PORTVERSION with DISTVERSION.
• Fix warnings from portclippy.
• Remove unnecessary MKDIR.
• Move install docs to do-install-DOCS-on goal.

PR: 293743