Restore and update developer documentation

We (I) accidentally deleted the developer documentation here: https://github.com/CDCgov/PyRenew/pull/579/files#diff-c8450e07d46c6c9963c36672ebe1f090462d345616ce2a5a5916a36ccc40272d

It should be restored to the website and the sphinx documentation should be updated for mkdocs.