build(deps): bump gatsby-plugin-mdx from 1.2.35 to 1.2.40 in /docs (#1386)

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 1.2.35 to 1.2.40.
- [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)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/package-lock.json

@@ -18,7 +18,7 @@
     "gatsby-plugin-emotion": "^4.3.10",
     "gatsby-plugin-google-analytics": "^2.3.13",
     "gatsby-plugin-manifest": "^2.4.27",
-    "gatsby-plugin-mdx": "^1.2.35",
+    "gatsby-plugin-mdx": "^1.2.40",
     "gatsby-plugin-netlify": "^2.3.13",
     "gatsby-plugin-netlify-cache": "^1.2.0",
     "gatsby-plugin-offline": "^3.2.27",