Simplify workflow files

[felixhekhorn]

Actually, having looked at the file, it is much too complicated I'd say - can't we split it into smaller snippets and scripts which are executed in the correct order inside the workflow?

Originally posted by @felixhekhorn in #310 (comment)

The comment originally refers to the release workflow, which arguably is the most complex script.

[cschwan]

It's a complex script because the tasks it performs are complicated, but I agree that we should try to split the tasks over a few files; at the very least that'll make editing a bit easier. We could split release.yml into the following additional files:

1. release-capi.yaml; generates the CAPI for Linux and macOS,
2. release-cli.yaml; generates the CLI for Linux and macOS,
3. release-cli-wheels.yaml; generate the CLI for Linux and macOS packaged as Python package installable by pip,
4. release-papi-wheels.yaml; generate the Python API wheels,

All the rest should probably stay inside release.yml, which must be there so we can manually trigger a release.