Update plugin docs

- Create separate files for each mixin
- Remove old "report" plugin docs

docs/extend/plugins/navigation.md

@@ -0,0 +1,25 @@
+---
+title: Navigation Mixin
+---
+
+## NavigationMixin
+
+Use the class constant `NAVIGATION` for a array of links that should be added to InvenTrees navigation header.  
+The array must contain at least one dict that at least define a name and a link for each element. The link must be formatted for a URL pattern name lookup - links to external sites are not possible directly. The optional icon must be a class reference to an icon (InvenTree ships with fontawesome 4 by default).
+
+``` python
+class MyNavigationPlugin(NavigationMixin, IntegrationPluginBase):
+
+    PLUGIN_NAME = "NavigationPlugin"
+
+    NAVIGATION = [
+        {'name': 'SampleIntegration', 'link': 'plugin:sample:hi', 'icon': 'fas fa-box'},
+    ]
+
+    NAVIGATION_TAB_NAME = "Sample Nav"
+    NAVIGATION_TAB_ICON = 'fas fa-plus'
+```
+
+The optional class constants `NAVIGATION_TAB_NAME` and `NAVIGATION_TAB_ICON` can be used to change the name and icon for the parent navigation node.
+
+