refactor(docs): add missing dev-guide to skeleton

docs/developers-guide.md

+# Developers Guide
+
+!!! todo
+    - small styleguide per used language
+    - need for unittests
+    - where to put what kind of class(core/systems package)
+    - how to commit (conventional changelog)
+    - GitLab workflow

mkdocs.yml

@@ -38,7 +38,7 @@ nav:
           - Ticketsystem: 'api/systems/gitlab/ticket-system.md'
           - Ticketsystem Builder: 'api/systems/gitlab/ticket-system-builder.md'
   - Developers Guide:
-    - Index: 'dummy.md'
+    - Index: 'developers-guide.md'
 
 theme:
   name: 'material'