Update plugin docs

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

docs/extend/plugins/urls.md

@@ -0,0 +1,23 @@
+---
+title: URLs Mixin
+---
+
+## UrlsMixin
+
+Use the class constant `URLS` for a array of URLs that should be added to InvenTrees URL paths or override the `plugin.setup_urls` function.  
+
+The array has to contain valid URL patterns as defined in the [django documentation](https://docs.djangoproject.com/en/stable/topics/http/urls/).
+
+``` python
+class MyUrlsPlugin(URLsMixin, IntegrationPluginBase):
+
+    PLUGIN_NAME = "UrlsMixin"
+
+    URLS = [
+        url(r'increase/(?P<location>\d+)/(?P<pk>\d+)/', self.view_increase, name='increase-level'),
+    ]
+```
+
+The URLs get exposed under `/plugin/{plugin.slug}/*` and get exposed to the template engine with the prefix `plugin:{plugin.slug}:` (for usage with the [url tag](https://docs.djangoproject.com/en/stable/ref/templates/builtins/#url)).
+
+