remove unsupported extractors from meltano.com docs

Currently, there are extractors listed at https://meltano.com/plugins/#extractors that are not available in the hosted Meltano UI. This is confusing.

We need to remove:

• CSV
• Carbon Intensity
• Marketo
• Mongo DB

If we are still going to allow users of the open source project to use them, we can note that there are additional unsupported extractors and link to documentation in the repo itself. The website should only be talking about what we have available for our target persona in the hosted version.