Publish documentation for latest release

[nawatts]

Currently, we publish documentation on every commit to master, so https://broadinstitute.github.io/gnomad_methods/ contains documentation for the latest development version. However, users are likely using the last PyPI release, which may lag behind development. We should possibly publish documentation only on releases or publish both a stable and development version of documentation.

[nawatts]

An example of confusion caused by this: https://discuss.hail.is/t/cannot-import-gnomad-utils-annotations-sor-from-sb/1920.

[gtiao]

Not sure if this is still relevant -- needs refinement

[ch-kr]

we now create pypi releases after each gnomAD version release -- can open a new ticket if this becomes an issue again