Trigger ReadTheDocs.org build only on successful publish

[helderco]

Related to #3668

Overview

Reference docs are being built when tags that start with sdk/python/v are detected.

Problem

The tag triggers a new PyPI deployment but that could fail. Since ReadTheDocs.org will publish based on git alone, and not on PyPI versions, you'll end up with a documented version that doesn't exist.

Solution

It makes more sense to trigger the build manually via a GET request to a custom webhook, inside mage's sdk:python:publish, and only after a successful upload to PyPI.