chore(deps): bump the documentation group with 2 updates

Bumps the documentation group with 2 updates: [sphinx](https://github.com/sphinx-doc/sphinx) and [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme).


Updates `sphinx` from 8.2.3 to 9.0.4
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v9.0.4/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v8.2.3...v9.0.4)

Updates `sphinx-rtd-theme` from 3.0.2 to 3.1.0
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@3.0.2...3.1.0)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 9.0.4
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: documentation
- dependency-name: sphinx-rtd-theme
  dependency-version: 3.1.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: documentation
...

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

Author: dependabot[bot]

docs/requirements.txt

@@ -125,15 +125,15 @@ six==1.17.0
     # via python-dateutil
 snowballstemmer==3.0.1
     # via sphinx
-sphinx==8.2.3
+sphinx==9.0.4
     # via
     #   numpydoc
     #   solarwindpy (pyproject.toml)
     #   sphinx-rtd-theme
     #   sphinxcontrib-bibtex
     #   sphinxcontrib-jquery
     #   sphinxcontrib-spelling
-sphinx-rtd-theme==3.0.2
+sphinx-rtd-theme==3.1.0
     # via solarwindpy (pyproject.toml)
 sphinxcontrib-applehelp==2.0.0
     # via sphinx