build(deps): bump gatsby-remark-images from 3.6.0 to 3.8.1 in /docs (#1601)

Bumps [gatsby-remark-images](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-remark-images) from 3.6.0 to 3.8.1.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/gatsby-remark-images@3.8.1/packages/gatsby-remark-images/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-remark-images@3.8.1/packages/gatsby-remark-images)

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Author: dependabot[bot]