chore: create and add plugin template

created with help by these Websites:
[Delicious Brains](https://deliciousbrains.com/wordpress-plugin-development-template-files/)
[Wordpress Plugin Boilerplate generator](https://wppb.me/)

rk-wichteln/admin/css/rk-wichteln-admin.css

@@ -0,0 +1,4 @@
+/**
+ * All of the CSS for your admin-specific functionality should be
+ * included in this file.
+ */