Use Read the Docs theme in local builds

[cjolowicz]

Configure Sphinx to use the Read the Docs theme.

This fixes a discrepancy between documentation builds on Read the Docs on one side (which use sphinx-rtd-theme by default), and builds on GitHub Actions and local builds on the other side (which default to Alabaster). As the documentation is hosted with Read the Docs, the discrepancy is resolved in favor of sphinx-rtd-theme, avoiding a breaking change.

Closes #564