Document hybrid database strategy (postgres and cassandra)

[Delapouite]

Hi

Currently the docker-compose files present in this repository tend to favor a "full-on" approach.
Using the DATABASE variable in the main .env file, the user can either takes the postgres OR the cassandra road.

Often, it is convenient to take the best of both worlds by storing the entities in postgres and the telemetries in cassandra as described in the official documentation as the "hybrid" storage:

DATABASE_TS_TYPE=cassandra
DATABASE_ENTITIES_TYPE=sql

Do you intend to evolve this repository so it can handle this common scenario? Thanks.