api-tutorial: example that demonstrates how to add new column to df + include clickable links to dracor frontend

[ingoboerner]

add example to demonstrate how to add a column with values to pd metadata_df.columns, e.g. a "play_link"
-->
API_URL (must to be manipulated or add DRACOR_URL/FRONTEND_URL) + "/{corpusname}/{playname}" should work ?

see also @lehkost comment in #8