build(deps): bump gatsby-remark-responsive-iframe in /docs (#1533)

Bumps [gatsby-remark-responsive-iframe](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-remark-responsive-iframe) from 2.4.12 to 2.6.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-remark-responsive-iframe/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-remark-responsive-iframe@2.6.0/packages/gatsby-remark-responsive-iframe)

Signed-off-by: dependabot[bot] <support@github.com>
Signed-off-by: Xiang Dai <long0dai@foxmail.com>
Co-authored-by: Xiang Dai <long0dai@foxmail.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/package-lock.json

@@ -33,7 +33,7 @@
     "gatsby-remark-external-links": "0.0.4",
     "gatsby-remark-images": "^3.3.29",
     "gatsby-remark-mermaid": "^2.1.0",
-    "gatsby-remark-responsive-iframe": "^2.4.12",
+    "gatsby-remark-responsive-iframe": "^2.6.0",
     "gatsby-source-filesystem": "^2.3.30",
     "gatsby-transformer-remark": "^2.8.28",
     "gatsby-transformer-sharp": "^2.5.15",