Docs: Downgrade theme to fix display bug

[dbanty]

Description

Insert your PR description here. Thanks for contributing to pyQuil! 🙂

Checklist

• The PR targets the rc branch (not master).
• The above description motivates these changes.
• There is a unit test that covers these changes.
• All new and existing tests pass locally and on the PR's checks.
• Parameters and return values have type hints with PEP 484 syntax.
• Functions and classes have useful Sphinx-style docstrings.
• All code follows Black style and obeys flake8 conventions.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using auto-close keywords.
• The changelog is updated, including author and PR number (@username, Test ignore #1234).

[rigetti-githubbot]

🎉 This PR is included in version 3.2.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀