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

Bumps [gatsby-remark-responsive-iframe](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-remark-responsive-iframe) from 5.11.0 to 5.13.0.
- [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@5.13.0/packages/gatsby-remark-responsive-iframe)

---
updated-dependencies:
- dependency-name: gatsby-remark-responsive-iframe
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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