Review documentation for release infrastructure

This PR should add documentation for the new CI release infrastructure including:

- graphs and descriptions of the various jobs involved in the release workflow
- address of the CI release servers used for the integration testing
- switch to https for the CI servers


NB: this PR is not expected to cover the new C++ release jobs. A separate PR will be opened for these once this is merged.

--no-rebase
':envvar:' again