build(deps): bump sphinx from 7.2.5 to 7.2.6 in /lib/spack/docs (#40029)

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.2.5 to 7.2.6.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](https://github.com/sphinx-doc/sphinx/compare/v7.2.5...v7.2.6)

---
updated-dependencies:
- dependency-name: sphinx
  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>

lib/spack/docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx==7.2.5
+sphinx==7.2.6
 sphinxcontrib-programoutput==0.17
 sphinx_design==0.5.0
 sphinx-rtd-theme==1.3.0