docs-deps(deps): bump sphinx-sitemap from 2.8.0 to 2.9.0 in /docs

Bumps [sphinx-sitemap](https://github.com/jdillard/sphinx-sitemap) from 2.8.0 to 2.9.0.
- [Release notes](https://github.com/jdillard/sphinx-sitemap/releases)
- [Changelog](https://github.com/jdillard/sphinx-sitemap/blob/master/CHANGELOG.rst)
- [Commits](jdillard/sphinx-sitemap@v2.8.0...v2.9.0)

---
updated-dependencies:
- dependency-name: sphinx-sitemap
  dependency-version: 2.9.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]