Publish config schema reference by generating it

[alexgarel]

We can use export_json_schema metho in app/config.py to generate a json schema file.

And then https://github.com/tomcollins/json-schema-static-docs to generate md files from them, that we could include in documentation hierarchy before calling mkdocs.

[alexgarel]

done.

see https://openfoodfacts.github.io/search-a-licious/users/ref-config/