Update sphinx requirement from <8.0,>=5.0 to >=5.0,<9.0 in /docs

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.0.2/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v5.0.0...v8.0.2)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-type: direct:production
...

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

docs/requirements-docs.txt

@@ -1,5 +1,5 @@
 # Requirements to build hpcss' documentation.
 
-sphinx >=5.0, <8.0
+sphinx >=5.0, <9.0
 sphinx-rtd-theme >=1.0, <3.0
 tomlkit >=0.1, < 1.0