build(deps): bump sphinx-rtd-theme in /lib/spack/docs

Bumps [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) from 1.3.0 to 2.0.0.
- [Changelog](https://github.com/readthedocs/sphinx_rtd_theme/blob/master/docs/changelog.rst)
- [Commits](readthedocs/sphinx_rtd_theme@1.3.0...2.0.0)

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

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

lib/spack/docs/requirements.txt

@@ -1,7 +1,7 @@
 sphinx==7.2.6
 sphinxcontrib-programoutput==0.17
 sphinx_design==0.5.0
-sphinx-rtd-theme==1.3.0
+sphinx-rtd-theme==2.0.0
 python-levenshtein==0.23.0
 docutils==0.18.1
 pygments==2.17.1