build(deps): bump gatsby-plugin-sitemap from 2.9.0 to 2.12.0 in /docs

Bumps [gatsby-plugin-sitemap](https://github.com/gatsbyjs/gatsby/tree/HEAD/packages/gatsby-plugin-sitemap) from 2.9.0 to 2.12.0.
- [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]>

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": "^2.14.1",
-    "gatsby-plugin-sitemap": "^2.9.0",
+    "gatsby-plugin-sitemap": "^2.12.0",
     "gatsby-remark-autolink-headers": "^2.10.0",
     "gatsby-remark-copy-linked-files": "^2.7.0",
     "gatsby-remark-embedder": "^4.1.0",