refactor(docs): api: core: remove classes which aren't specifically mentioned in user docs

docs/api/core/filter.md

-# Filter
-
-!!! todo
-    write usage api docs

docs/api/core/registered-systems.md

-# Registered Systems
-
-!!! todo
-    write usage api docs

docs/api/core/ticket-assignee.md

-# Ticket Assignee
-
-!!! todo
-    write usage api docs

docs/api/core/ticket-builder.md

-# Ticket-Builder
-
-!!! todo
-    write usage api docs

docs/api/core/ticket-system-builder.md

-# Ticketsystem-Builder
-
-!!! todo
-    write usage api docs

mkdocs.yml

@@ -20,14 +20,9 @@ nav:
   - API:
     - Index: 'api/index.md'
     - Core:
-      - Filter: 'api/core/filter.md'
       - Logging: 'api/core/logging.md'
-      - Registered Systems: 'api/core/registered-systems.md'
       - Ticket: 'api/core/ticket.md'
-      - Ticket Assignee: 'api/core/ticket-assignee.md'
-      - Ticket Builder: 'api/core/ticket-builder.md'
       - Ticketsystem: 'api/core/ticket-system.md'
-      - Ticketsystem Builder: 'api/core/ticket-system-builder.md'
     - Exceptions: 'api/exceptions.md'
     - Systems:
         - GitLab: