build(deps): update sphinx requirement from >=7 to >=8.1.3

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/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v7.0.0...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 8.1.3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

doc/requirements.txt

@@ -1,4 +1,4 @@
 .[docs,cpu,torch]
 # https://github.com/mcmtroffaes/sphinxcontrib-bibtex/issues/309
 docutils!=0.18.*,!=0.19.*
-sphinx>=7
+sphinx>=8.1.3