build(deps-dev): bump mkdocs-section-index from 0.3.8 to 0.3.9

Bumps [mkdocs-section-index](https://github.com/oprypin/mkdocs-section-index) from 0.3.8 to 0.3.9.
- [Release notes](https://github.com/oprypin/mkdocs-section-index/releases)
- [Commits](oprypin/mkdocs-section-index@v0.3.8...v0.3.9)

---
updated-dependencies:
- dependency-name: mkdocs-section-index
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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