Added guide for setting up Next.js with MDX

[molebox]

This PR adds a new section on using MDX. It includes a guide for setting up Next.js with @next/mdx

Bug

• Related issues linked using fixes: Add MDX section to Advance features #29597
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using Fixes: https://github.com/vercel/product/issues/35
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running yarn lint

[styfle]

Lint is failing because manifest file needs to be updated

No missing paths in errors/manifest.json
Missing paths in docs/manifest.json:
docs/advanced-features/using-mdx.md
Error: missing/incorrect manifest items detected see above
    at main (/home/runner/work/next.js/next.js/scripts/check-manifests.js:61:11)

[molebox]

Lint is failing because manifest file needs to be updated

No missing paths in errors/manifest.json
Missing paths in docs/manifest.json:
docs/advanced-features/using-mdx.md
Error: missing/incorrect manifest items detected see above
    at main (/home/runner/work/next.js/next.js/scripts/check-manifests.js:61:11)

Ah, thanks @styfle! Just added it

[molebox]

What say thee @styfle @leerob can we land this?

[leerob]

LGTM after last comment

[leerob]

It looks like we didn't get this spacing adjusted - maybe it needs to be js instead of md?

[molebox]

Hmm, that could be it! Rather odd. We should adjust the config for that, as I pulled this out so js makes sense, but it was in the context of an md file, so should format.