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

docs/api/core/ticket.md

 # Ticket
 
-!!! todo
-    write usage api docs
+A ticket instance represents one ticket of the connected space.
+
+## get/create ticket
+
+The class is not publicly constructable,
+as it always belongs to a `TicketSystem` object.
+
+Getting an instance for a existing ticket,
+or creating a new one,
+is done from a `TicketSystem` instance.
+[->For details see here <-](./ticket-system.md)
+## reading data
+
+There are various getters available.
+Below the globally defined ones are listed,
+each system specific implementation is free to provide additional ones.
+
+!!! warning
+    systems are not guaranteed to support all data,
+    a global getter is defined for.
+    Therefore it can happen, you get an null value back
+    or exception thrown in this case.
+
+| method             | return                |
+| :----------------- | :-------------------- |
+| `getAssignees()`   | `Set<TicketAssignee>` |
+| `getDescription()` | `String`              |
+| `getId()`          | `String`              |
+| `getLabels()`      | `Set<String>`         |
+| `getParent()`      | `TicketSystem`        |
+| `getTitle()`       | `String`              |
+| `isOpen()`         | `boolean`             |
+
+## changing data
+
+You can change data of supported fields through dedicated setters.
+Every changed field is flagged as changed.
+To persist the changes to the connected space,
+you need to call `save()`.
+This forms an update request for all flagged fields
+and returns a new instance of `Ticket`,
+formed off the answer from the connected system.
+
+Below the globally defined setters are listed,
+each system implementation is free to offer more.
+
+| method                                              | description                                                     |
+| :-------------------------------------------------- | :-------------------------------------------------------------- |
+| `addAssignee(String)`<br/>`removeAssignee(String)`  | adds/removes assignee, check system specific part, what to pass |
+| `addLabel(String)`                                  | add label                                                       |
+| `clearAssignees()`                                  | remove all assignees                                            |
+| `open()`<br/>`close()`                              | opens/closes the ticket                                         |
+| `setDescription(String)`                            |                                                                 |
+| `setLabels(Set<String>)`<br/>`setLabels(String[])`  | replace all labels with the passed ones                         |
+| `setTitle(String)`                                  |                                                                 |