[docs][workflows] Centralizing MDX docs on Gatsbyjs.org

Part of the Top 25 Learning Workflows initiative. See #13708 for the meta issue that this issue falls under.

Description:

In an effort to centralize docs for MDX under the Gatsbyjs.org docs, all guides should be moved from the gatsby-mdx docs to .org. We should add createRedirects on the gatsby-mdx page as well to point users to Gatsby’s docs where documentation will continue for what is now gatsby-plugin-mdx.

~The name of the reference guides on MDX has been a question, should “Adding Components to Content with MDX” be changed to something that instead mentions adding JavaScript?~

• add new page in Gatsby API section of the docs sidebar for MDX
• move docs from gatsby-mdx.netlify.com to .org
• create redirects from gatsby-mdx.netlify.com to corresponding pages on .org
• update references to https://gatsby-mdx.netlify.com/ to new docs on .org
• update references to gatsby-mdx to official plugin gatsby-plugin-mdx
• clarify docs on using frontmatter and imports in the same file

Related:

#14258

[gillkyle]

Yeah that’s a good point @robinmetral, I know MDX uses rehype and remark internally (see here) so trying to push people towards MDX seems like a good idea there, though I think it’s great to have that documentation still the same.

[marcysutton]

I checked in with Amberley and Kyle Gill, and it sounds like this one is indeed done now. Any remaining improvements can be handled in new issues and PRs. Thanks so much!