build(deps): bump gatsby-transformer-yaml from 2.4.11 to 2.8.0 in /docs

Bumps [gatsby-transformer-yaml](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-transformer-yaml) from 2.4.11 to 2.8.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-yaml/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-transformer-yaml@2.8.0/packages/gatsby-transformer-yaml)

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

Author: dependabot[bot]