Document how to add a new app to Stackspin

The local-it devs already asked for a guide so we should document this properly now that we have upgraded to flux2. Best would be to do this while adding new apps (prototype-funding#15).

• Add flux2 heath-check to flux2/cluster/base/?.yaml (see https://open.greenhost.net/openappstack/openappstack/-/merge_requests/436/diffs#baa39bf911458b2a7c6f43e63bb91e315adf2cf8_24_25)