build(deps): update mkdocs-section-index requirement in /pywry/docs

Updates the requirements on [mkdocs-section-index](https://github.com/oprypin/mkdocs-section-index) to permit the latest version.
- [Release notes](https://github.com/oprypin/mkdocs-section-index/releases)
- [Commits](oprypin/mkdocs-section-index@v0.3.0...v0.3.12)

---
updated-dependencies:
- dependency-name: mkdocs-section-index
  dependency-version: 0.3.12
  dependency-type: direct:production
...

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

Author: dependabot[bot]

pywry/docs/requirements.txt

@@ -10,7 +10,7 @@ griffe>=1.0.0
 # For cross-references
 mkdocs-gen-files>=0.5.0
 mkdocs-literate-nav>=0.6.0
-mkdocs-section-index>=0.3.0
+mkdocs-section-index>=0.3.12
 
 # Needed for building docs (dependencies of pywry)
 pydantic>=2.0