Automate documentation publishing

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Doc publishing is currently done completely manually. Ideally, once any update is done to master on any of the repos (npgsql, EF Core provider, EF6 provider) the change would automatically go live.

This would mean two things:

1. Continuous deployment on each of the 3 repos which detects changes on the master branch, and updates the appropriate git submodule on the doc repo. Pushing this would trigger:
2. Continuous deployment on https://github.com/npgsql/doc which automatically generates the site on each commit and pushes it to github (deploying live)

We should wait and see if we're transitioning from Appveyor/Travis to Azure Pipelines before attacking this.

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This is now done on github actions.