adding a gatsby-plugin-markdown-output (not for ingesting markdown, but for outputting markdown)

[Pomax]

It's currently possible to get HTML (obviously) or JSON (using gatsby-plugin-markdown-output) as URL output, but it would be super useful to also be able to ask for a markdown output, so that systems that need to "read the docs" can do that in markdown rather than only HTML or JSON.

Note that this is not about ingesting markdown, but about taking the final page content and adding a way to output that as markdown text with a text/markdown content type.