docs: added note about the difference between groups and extras

docs/managing-dependencies.md

@@ -50,6 +50,14 @@ pytest-mock = "*"
 ```
 {{% /note %}}
 
+{{% note %}}
+Dependency groups, other than the implicit `main` group, must only contain dependencies you need in your development
+process. Installing them is only possible by using Poetry.
+
+To declare a set of dependencies, which add additional functionallity to the project during runtime,
+use [extras]({{< relref "pyproject#extras" >}}) instead. Extras can be installed by the end user using `pip`.
+{{% /note %}}
+
 {{% note %}}
 **A note about the `dev-dependencies` section**