Fix Read the Docs builds

[kurtmckee]

The documentation is failing to build on Read the Docs.

This PR:

• Introduces a Read the Docs configuration
• Locks the documentation build requirements to ensure reproducible builds
• Fixes two trivial warnings that occur when building the docs

This work doesn't address #317 but makes it trivial to reproduce warnings and fix them, since all dependencies are locked.

If #344 merges, the documentation requirements can migrate to that same requirements management framework, and documentation builds can be tested/linted as a part of the test suite using tox.

[corydolphin]

Thank you @kurtmckee for the contribution! This looks great. Merging.

[kurtmckee]

Thanks Cory!

I'm not seeing this trigger a new build on Read the Docs, though; it looks like the main branch isn't getting picked up as a version to build (link to listed versions). Would you double-check the settings for the project?