Update sphinx-rtd-theme requirement from ~=1.3.0 to ~=2.0.0 (#158)

Updates the requirements on [sphinx-rtd-theme](https://github.com/readthedocs/sphinx_rtd_theme) to permit the latest version.
- [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
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-docs.txt

@@ -13,4 +13,4 @@ ipython ~=7.34.0
 numpydoc ~=1.6
 sphinx ~=7.2.6
 sphinx-copybutton ~=0.5.2
-sphinx_rtd_theme ~=1.3.0
+sphinx_rtd_theme ~=2.0.0