build(deps): bump gatsby-plugin-mdx from 2.2.0 to 2.6.0 in /docs

Bumps [gatsby-plugin-mdx](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-mdx) from 2.2.0 to 2.6.0.
- [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@2.6.0/packages/gatsby-plugin-mdx)

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

Author: dependabot[bot]