build(deps-dev): bump sphinx-immaterial from 0.12.5 to 0.13.8

Bumps [sphinx-immaterial](https://github.com/jbms/sphinx-immaterial) from 0.12.5 to 0.13.8.
- [Release notes](https://github.com/jbms/sphinx-immaterial/releases)
- [Commits](jbms/sphinx-immaterial@v0.12.5...v0.13.8)

---
updated-dependencies:
- dependency-name: sphinx-immaterial
  dependency-version: 0.13.8
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

pyproject.toml

@@ -60,7 +60,7 @@ dev = [
 
 docs = [
     "sphinx",
-    "sphinx-immaterial==0.12.5",
+    "sphinx-immaterial==0.13.8",
     "sphinx_copybutton",
     "sphinx-autobuild",
     "tornado>=6.3.3"