docs: Identify and clean up references to broken or unmaintained plugins

There are currently several plugins referenced in the documentation and ecosystem that appear to be outdated, unmaintained, or no longer functional. This makes it harder for users to find reliable tools and contributes to confusion about what's actively supported.

Objective

We should audit the list of known plugins (in the docs) and identify:

• Plugins that are broken or no longer work with current Beets versions
• Plugins that are no longer maintained (e.g., stale repos, no recent commits/issues)
• Any links in the docs or plugin lists that are dead or outdated
• The sidebar of the plugins in the docs is way to long! We should put them into the same categories as in the main page.

This issue was prompted by #5779 noting that some plugins, while historically useful (like beets-copyartifacts), may no longer be actively supported or even working with the current beets version.