feat(docs): Clarify gatsby-plugin-mdx requirements for gatsby-source-filesystem

[yamalight]

Description

Adds an example code and additional note explaining that mdx plugin needs to have filesystem pre-processing files.

Related Issues

#15594

[jonniebigodes]

@yamalight fine on my end. I'll leave to core team to review it and possibly merge it.

[marcysutton]

Ooh, great addition! The PR needs to be updated to use the current plugin version: gatsby-plugin-mdx and then it should be good to go.

Thanks for working on this!

[yamalight]

@marcysutton should be updated now I think 🤔

[johno]

Thanks for this! Added a couple suggestions and had a question for you.

[johno]

✨ lgtm!

[johno]

Thanks @yamalight!

[gatsbot]

Holy buckets, @yamalight — we just merged your PR to Gatsby! 💪💜

Gatsby is built by awesome people like you. Let us say “thanks” in two ways:

1. We’d like to send you some Gatsby swag. As a token of our appreciation, you can go to the Gatsby Swag Store and log in with your GitHub account to get a coupon code good for one free piece of swag. We’ve got Gatsby t-shirts, stickers, hats, scrunchies, and much more. (You can also unlock even more free swag with 5 contributions — wink wink nudge nudge.) See gatsby.dev/swag for details.
2. We just invited you to join the Gatsby organization on GitHub. This will add you to our team of maintainers. Accept the invite by visiting https://github.com/orgs/gatsbyjs/invitation. By joining the team, you’ll be able to label issues, review pull requests, and merge approved pull requests.

If there’s anything we can do to help, please don’t hesitate to reach out to us: tweet at @gatsbyjs and we’ll come a-runnin’.

Thanks again!