cirrus.yml: automatic skips based on source

As we want to get rid of the special titles convert the existing skips
to the only_if condition, this makes it more readable as we do not need
to negate so much.

Then add similar conditions for all test tasks, this removes the need to
a special title such as CI:DOCS as the logic is smart enough to only
docs changes when no source code was changed.

Update the documentation for the new logic and no longer point
contributors to the CI:DOCS title as it is gone now.

There is a bunch of duplication in the rules as yaml doesn't allow us to
share only parts of a string. To prevent unwanted drift a test case in
contrib/cirrus/cirrus_yaml_test.py is added to ensure all conditions
follow the same base ruleset.

Signed-off-by: Paul Holzinger <[email protected]>