Update bundler.io with Bundler 4 release

Note: This technically belongs in https://github.com/rubygems/bundler-site but I wanted to track it here to give it the most visibility.

bundler.io is the canonical source for Bundler documentation, but it has not been updated for the Bundler 4 release. We should follow the previous release steps to ensure the site reflects the most current release and has up-to-date documentation.