build(deps): bump gatsby-plugin-sharp from 2.6.25 to 2.6.31 in /docs (#1354)

Bumps [gatsby-plugin-sharp](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sharp) from 2.6.25 to 2.6.31.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-sharp/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-sharp)

Signed-off-by: dependabot[bot] <[email protected]>
Signed-off-by: Xiang Dai <[email protected]>
Co-authored-by: Xiang Dai <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]