PluginDescriptor page leads to non-existent XSD

### Bug description

On the page https://maven.apache.org/ref/4.0.0-alpha-4/maven-plugin-api/plugin.html

Clicking on this link "An XSD is available at:https://maven.apache.org/xsd/plugin-1.1.0.xsd" goes to "Page not found" page

### Maven site URL where bug exists

https://maven.apache.org/ref/4.0.0-alpha-4/maven-plugin-api/plugin.html

Start with the Maven site source for the page at https://maven.apache.org/ref/4.0.0-alpha-4/maven-plugin-api/plugin.html and search for the linked XSD URL. Verify the intended destination and confirm the page's XSD link no longer leads to a 404 page.

Written by the indexing model from the issue text.