Update plugin docs

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

docs/extend/plugins/settings.md

@@ -0,0 +1,48 @@
+---
+title: Settings Mixin
+---
+
+## SettingsMixin
+
+The *SettingsMixin* allows the plugin to save and load persistent settings to the database.
+
+- Plugin settings are stored against the individual plugin, and thus do not have to be unique
+- Plugin settings are stored using a "key:value" pair
+
+Use the class constant `SETTINGS` for a dict of settings that should be added as global database settings.  
+
+The dict must be formatted similar to the following sample. Take a look at the settings defined in `InvenTree.common.models.InvenTreeSetting` for all possible parameters.
+
+
+``` python
+class PluginWithSettings(SettingsMixin, IntegrationPluginBase):
+    
+    PLUGIN_NAME = "PluginWithSettings"
+    
+    SETTINGS = {
+        'API_ENABLE': {
+            'name': 'API Functionality',
+            'description': 'Enable remote API queries',
+            'validator': bool,
+            'default': True,
+        },
+        'API_KEY': {
+            'name': 'API Key',
+            'description': 'Security key for accessing remote API',
+            'default': '',
+        },
+        'API_URL': {
+            'name': _('API URL'),
+            'description': _('Base URL for remote server'),
+            'default': 'http://remote.url/api',
+        },
+    }
+```
+
+This mixin defines the helper functions `plugin.get_setting` and `plugin.set_seting` to access all plugin specific settings:
+
+```python
+api_url = self.get_setting('API_URL')
+```
+
+