build(deps): bump gatsby-transformer-sharp from 2.5.15 to 2.9.0 in /docs

Bumps [gatsby-transformer-sharp](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-transformer-sharp) from 2.5.15 to 2.9.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-sharp/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/gatsby-transformer-sharp@2.9.0/packages/gatsby-transformer-sharp)

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

Author: dependabot[bot]