build(deps): bump gatsby-plugin-manifest from 2.4.27 to 2.9.1 in /docs

Bumps [gatsby-plugin-manifest](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-manifest) from 2.4.27 to 2.9.1.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/gatsby-plugin-manifest@2.9.1/packages/gatsby-plugin-manifest/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-manifest@2.9.1/packages/gatsby-plugin-manifest)

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

Author: dependabot[bot]