add documentation about ingestion

[vincentsarago]

with #144 we will get a new ingestion service deployed by default.

TODO

• add simple documentation in the docs about what it is and how to use it
• add example how to use it
• add documentation about the authentification method it uses

[emileten]

would it make sense to try to add it as a service in the local deployments ? I am not even sure it would work, I now realize how AWS-dependent it is.

[drwelby]

Is there more info on the ingestor? It's quite topical right now and I can probably put some docs together around it.