build(deps): bump gatsby-plugin-sitemap from 2.4.14 to 2.9.0 in /docs (#1605)

Bumps [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap) from 2.4.14 to 2.9.0.
- [Release notes](https://github.com/gatsbyjs/gatsby/releases)
- [Changelog](https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-sitemap/CHANGELOG.md)
- [Commits](https://github.com/gatsbyjs/gatsby/commits/[email protected]/packages/gatsby-plugin-sitemap)

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

@@ -26,7 +26,7 @@
     "gatsby-plugin-react-svg": "^3.0.0",
     "gatsby-plugin-robots-txt": "^1.5.1",
     "gatsby-plugin-sharp": "^2.6.40",
-    "gatsby-plugin-sitemap": "^2.4.14",
+    "gatsby-plugin-sitemap": "^2.9.0",
     "gatsby-remark-autolink-headers": "^2.3.13",
     "gatsby-remark-copy-linked-files": "^2.3.12",
     "gatsby-remark-embedder": "^4.0.0",