build(deps): bump gatsby-plugin-mdx from 3.13.0 to 3.15.2 in /docs

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 3.13.0 to 3.15.2.
- [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/[email protected]/packages/gatsby-plugin-mdx)

---
updated-dependencies:
- dependency-name: gatsby-plugin-mdx
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

Diff for: docs/package-lock.json

@@ -17,7 +17,7 @@
     "gatsby-plugin-emotion": "^7.13.0",
     "gatsby-plugin-google-analytics": "^4.13.0",
     "gatsby-plugin-manifest": "^4.15.1",
-    "gatsby-plugin-mdx": "^3.13.0",
+    "gatsby-plugin-mdx": "^3.15.2",
     "gatsby-plugin-netlify": "^4.3.1",
     "gatsby-plugin-netlify-cache": "^2.0.0",
     "gatsby-plugin-offline": "^5.13.0",