update docs API to include calculate_markup

Author: Carlos

docs/api.md

@@ -39,4 +39,10 @@
     options:
         members:
             - calculate_compound_interest
+        show_root_toc_entry: False
+
+::: dinero.tools.calculate_markup
+    options:
+        members:
+            - calculate_markup
         show_root_toc_entry: False