chore: update showcases

[SethFalco]

This is a 🔦 documentation change.

Summary

When browsing the showcase, there was a broken link. While I was at it, I figured it was worth checking some other links too:

• Replace links to where they redirect if a redirect occurs. (Excluding localization.)
• Remove locale from the URL, so it uses the preferred language of the user.
• Add a trailing slash on base URLs for consistency.
• Remove dead links.

[mattr-]

Thank you for updating all of these! ❤️

@jekyllbot: merge +docs