refactor(docs): api: systems: gitlab: add hint about local filtrs to...

refactor(docs): api: systems: gitlab: add hint about local filtrs to ticket-system, remove seperate filter file

docs/api/systems/gitlab/filter.md

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

docs/api/systems/gitlab/ticket-system.md

@@ -107,6 +107,12 @@ mostly the same like building by URI applies.
     GitLab implementation supports the fields `id`, `username` and `full name`.
     If ticket has no assignee, assignee field stays `null`.
 
+From the filters shown on the global `TicketSystem` docs,
+the ones for regex-matching are not natively supported by the GitLab API.
+If such a filter is used,
+this get's done locally after receiving the response from GitLab,
+before handing it to the user.
+
 ## get single ticket
 
 !!! info

mkdocs.yml

@@ -26,7 +26,6 @@ nav:
     - Exceptions: 'api/exceptions.md'
     - Systems:
         - GitLab:
-          - Filter: 'api/systems/gitlab/filter.md'
           - Ticket: 'api/systems/gitlab/ticket.md'
           - Ticketsystem: 'api/systems/gitlab/ticket-system.md'
   - Developers Guide: