chore(deps-dev): Bump sphinx-substitution-extensions

Bumps [sphinx-substitution-extensions](https://github.com/adamtheturtle/sphinx-substitution-extensions) from 2026.1.12 to 2026.6.17.
- [Release notes](https://github.com/adamtheturtle/sphinx-substitution-extensions/releases)
- [Changelog](https://github.com/adamtheturtle/sphinx-substitution-extensions/blob/main/CHANGELOG.rst)
- [Commits](adamtheturtle/sphinx-substitution-extensions@2026.01.12...2026.06.17)

---
updated-dependencies:
- dependency-name: sphinx-substitution-extensions
  dependency-version: 2026.6.17
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

pyproject.toml

@@ -74,7 +74,7 @@ optional-dependencies.dev = [
     "sphinx-copybutton==0.5.2",
     "sphinx-lint==1.0.2",
     "sphinx-pyproject==0.3.0",
-    "sphinx-substitution-extensions==2026.1.12",
+    "sphinx-substitution-extensions==2026.6.17",
     "sphinxcontrib-spelling==8.0.2",
     # ``sphinxcontrib-towncrier`` renders unreleased news fragments
     # into docs/source/unreleased.rst during Sphinx builds.