build(deps): bump gatsby-plugin-mdx from 3.6.0 to 3.9.0 in /docs

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 3.6.0 to 3.9.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-mdx@3.9.0/packages/gatsby-plugin-mdx)

---
updated-dependencies:
- dependency-name: gatsby-plugin-mdx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

docs/package.json

@@ -17,7 +17,7 @@
     "gatsby-plugin-emotion": "^7.7.0",
     "gatsby-plugin-google-analytics": "^4.6.0",
     "gatsby-plugin-manifest": "^4.7.0",
-    "gatsby-plugin-mdx": "^3.6.0",
+    "gatsby-plugin-mdx": "^3.9.0",
     "gatsby-plugin-netlify": "^4.1.0",
     "gatsby-plugin-netlify-cache": "^2.0.0",
     "gatsby-plugin-offline": "^5.6.0",