Maintain and serve a PGXN web site

Changes for 0.23.7 - 2024-02-22T20:50:06Z

• Removed duplicate CC license image and link on the /art page.
• Moved the generation of HTML from Multimarkdown files from build time to release time, and switched from Text::Multimarkdown to the `multimarkdown` CLI.
• Tweaked the CSS for definition lists to open up space a bit more between terms and definitions.