Resolve "readthedocs build fails"

Closes #608 (closed).

• Add doc/requirements.txt. This is the only strictly required fix.
• Add .readthedocs.yaml so that any readthedocs configuration change goes through the usual review. More on readthedocs configuration here.
• The newly added fail_on_warning: true makes the build fail because of WARNING: unknown mimetype for _static/arch.drawio, ignoring. I've moved arch.drawio out of static and added a rake task to build the PNG.

Git-tagged as v0.22.0-fix. Documentation is now live: https://stork.readthedocs.io/en/v0.22.0-fix/.