CI: Build docs website on develop, API on master

Both docs, website and API, are built on `master` only. While API reference should reflect the latest published release, docs website do not. 

This issue is to track progress on updating [Github Actions docs workflow](https://github.com/awslabs/aws-lambda-powertools-python/blob/develop/.github/workflows/python_docs.yml) to use a conditional check on branch.