custom markdown renderer

Rendering Markdown content (for description texts) is currently baked into `@graphiql/react`. There we have an instance of `markdown-it` that takes care of all the markdown rendering.

@acao mentioned [here](https://github.com/graphql/graphiql/pull/2582#discussion_r928282116) that supporting custom markdown renders might be a valuable addition. If you stumble upon this issue, feedback is very welcome as we haven't yet seen any issue around this in our repo!

In terms of code structure, we could also move the `markdown-it` instance to `@graphiql/toolkit` as rendering markdown is not React specific.