Support a flat ndjson dir layout as well as a hierarchical one

[mikix]

Previously, --load-ndjson-dir would only look for an ETL output-style format like this:

dir/condition/*.ndjson dir/patient/*.ndjson

But now it will also look for flat files as well (i.e. ETL input-style format) like this:

dir/1.Patient.ndjson dir/Patient.october.ndjson dir/Patient.ndjson

This will make it nicer to use the --load-ndjson-dir flow when you are working on ndjson files directly, without going through Cumulus ETL first.

Checklist

• [x] Consider if documentation (like in docs/) needs to be updated
• [x] Consider if tests should be added
• [x] Update template repo if there are changes to study configuration

[mikix]

I think this needs some documentation tweaks for the public site

Added a new commit to update that page (the only doc page that mentioned --load-ndjson-dir) to basically drop the ETL step entirely.