Update Hub main page to just have Meltano Plugins, maintain separate Singer pages

From https://gitlab.com/meltano/hub/-/issues/258,

• on the main page there should only be the plugin types Meltano knows about. Extractors should be 300+ etc.
• We will want to maintain the https://hub.meltano.com/singer/ page but don't need to have a link in the header for it.