Build: Bump mkdocs-section-index from 0.3.8 to 0.3.9 (apache#696)

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:production
  update-type: version-update:semver-patch
...

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

mkdocs/requirements.txt

@@ -25,4 +25,4 @@ mkdocs-autorefs==1.0.1
 mkdocs-gen-files==0.5.0
 mkdocs-material==9.5.20
 mkdocs-material-extensions==1.3.1
-mkdocs-section-index==0.3.8
+mkdocs-section-index==0.3.9