CI: switch to Github

[hagenw]

Adjusts the CI pipeline to Github by:

• adding an Action to run tests for Python 3.9, 3.10, 3.11 (.github/workflows/test.yml)
• adding an Action to test building the docs on Python 3.10 (.github/workflows/doc.yml)
• adding an Action to check for correct code formatting (.github/workflows/linter.yml)
• adding an Action to publish the Python package to PyPI (.github/workflows/publish.yml)
• updating CONTRIBUTING.rst to reflect the new URL of the repository and removing leftover discussions of C++ related stuff
• replacing testdata by emodb in the tests
• updating the docstring of auglib.Augment to use emodb instead of testdata

The CI jobs do also cache the data we load with audb so we do not need to load it every time from our public Artifactory server, see https://github.com/audeering/auglib/actions/caches

[audeerington]

Looks good to me!