Documentation integration (#4653)

* Add documentation under docs/ directory

* Add CI workflow for mkdocs configuration checking

* Add documentation issue template

* update pip-tools?

* Update .gitignore files

* Fix .gitignore rules

* Improve release notes page

* remove references to old repo

docs/docs/extend/plugins/action.md

@@ -0,0 +1,18 @@
+---
+title: Action Plugins
+---
+
+## ActionMixin
+
+Arbitrary "actions" can be called by POSTing data to the `/api/action/` endpoint. The POST request must include the name of the action to be performed, and a matching ActionPlugin plugin must be loaded by the server. Arbitrary data can also be provided to the action plugin via the POST data:
+
+```
+POST {
+    action: "MyCustomAction",
+    data: {
+        foo: "bar",
+    }
+}
+```
+
+For an example of a very simple action plugin, refer to `/InvenTree/plugin/builtin/action/simpleactionplugin.py`