refactor(docs): api: core: write 'Logging'

docs/api/core/logging.md

 # Logging
 
-!!! todo
-    write usage api docs
+The library uses the built-in logger from `java.util.logging`.
+It configures to not use the parent logger and places a new ConsoleHandler
+on static initialization. This outputs the log in formatted lines like:
+```text
+[2020-11-27T14:40:26+0100] [WARNING] [de.hftstuttgart.unifiedticketing.core.Logging.test] calling from app
+```
+
+This will _not_ change visual presentation based on any localization.
+
+## change log-level
+
+Default is to use _INFO_-level.
+You can change the used level:
+```java
+Logging.setLevel(Level);
+```
+
+The parameter passed is of type `java.util.logging.Level`.
+
+## test logging output
+
+To test if the logging as such works,
+you can pass a test string:
+```java
+Logging.test("test message");
+```
+
+This message will be logged by the `Logging`-class own Logger,
+once per level.
+
+## set custom formatter
+
+If you need another format,
+you can replace the default formatter of this lib by calling:
+```java
+Logging.setFormatter(Formatter);
+```
+
+This formatter has to be of type `java.util.logging.Formatter`