docs: get_kind() note added to "Systems of Quantities" chapter

docs/users_guide/framework_basics/systems_of_quantities.md

@@ -392,4 +392,9 @@ static_assert(same_type<kind_of<isq::length> / isq::time, isq::length / isq::tim
 
     Only a root quantity from the hierarchy tree or the one marked with `is_kind` specifier
     in the `quantity_spec` definition can be put as a template parameter to the `kind_of`
-    specifier. For example, `kind_of<isq::width>` will fail to compile.
+    specifier. For example, `kind_of<isq::width>` will fail to compile. However, we can call
+    `get_kind(q)` to obtain a kind of any quantity:
+
+    ```cpp
+    static_assert(get_kind(isq::width) == kind_of<isq::length>);
+    ```