Migrate readthedocs theme to pydata_sphinx_theme

[mmcauliffe]

PyData Sphinx theme is a more modern Bootstrap-based theme than the current readthedocs default theme.

MFA docs can be used as reference for implementation: https://github.com/MontrealCorpusTools/Montreal-Forced-Aligner/tree/main/docs/source, along with mfa-models docs too: https://github.com/MontrealCorpusTools/mfa-models/tree/main/docs/source, but they're a bit more complicated with how models and dictionaries are set up for tagging and searchability.

Other related infrastructure files:

• https://github.com/MontrealCorpusTools/Montreal-Forced-Aligner/blob/main/.readthedocs.yaml
• https://github.com/MontrealCorpusTools/Montreal-Forced-Aligner/blob/main/rtd_environment.yml