build(deps): bump gatsby-transformer-sharp in /docs (golangci#1402)

Bumps [gatsby-transformer-sharp](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-transformer-sharp) from 2.5.13 to 2.5.15.
- [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/[email protected]/packages/gatsby-transformer-sharp)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docs/package-lock.json

@@ -36,7 +36,7 @@
     "gatsby-remark-responsive-iframe": "^2.4.12",
     "gatsby-source-filesystem": "^2.3.30",
     "gatsby-transformer-remark": "^2.8.28",
-    "gatsby-transformer-sharp": "^2.5.13",
+    "gatsby-transformer-sharp": "^2.5.15",
     "gatsby-transformer-yaml": "^2.4.11",
     "polished": "^3.6.6",
     "prism-react-renderer": "^1.1.1",