Clean up Sphinx docs workflow

[riley-harper]

Now that we are automatically generating the Sphinx docs on pushes to main, we can clean up the previous workflow by

• Removing the docs/ directory.
• Updating doc/developer.md with the new workflow instructions.
• Updating the sphinx-docs/Makefile so that it doesn't copy the build files to docs/.
• Pinning Sphinx and Recommonmark less closely in pyproject.toml.

This closes #128.

[joegrover]

👍