build(deps): bump gatsby-plugin-manifest from 4.13.0 to 4.15.1 in /docs

Bumps [gatsby-plugin-manifest](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-manifest) from 4.13.0 to 4.15.1.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/gatsby-plugin-manifest@4.15.1/packages/gatsby-plugin-manifest/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-plugin-manifest@4.15.1/packages/gatsby-plugin-manifest)

---
updated-dependencies:
- dependency-name: gatsby-plugin-manifest
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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