GitHub workflows

[peterdesmet]

1. test.yml: run for every commit, one Python version / for PR, all Python versions
2. documentation.yml: for every commit to main, run test, run Sphinx, push to gh-pages branch
3. release.yml: run for commits with a tag, run tests for all Python version, push to Pypi

[stijnvanhoey]

Additional:

• remove the scheduled version of the test-run
• do not pin the tox version (or pin it everywhere)
• make sure the pip-compile update is done
• make sure the unit tests are checked against pinned version for app deployment
• pip-tools integration to provide deployable version with fixed dependency set
• deployment sphinx website and pypi (key setup,...)