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/gatsby-plugin-sharp@2.6.31/packages/gatsby-plugin-sharp)

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Signed-off-by: Xiang Dai <long0dai@foxmail.com>
Co-authored-by: Xiang Dai <long0dai@foxmail.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]