build(deps): bump sphinx from 7.4.7 to 8.1.3 in /lib/spack/docs

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 7.4.7 to 8.1.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/master/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.4.7...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

lib/spack/docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx==7.4.7
+sphinx==8.1.3
 sphinxcontrib-programoutput==0.17
 sphinx_design==0.6.1
 sphinx-rtd-theme==2.0.0