Documentation mentions no_db option that doesn't exist

[nachog00]

3. Documentation mentions no_db option that doesn't exist

Location: docs/use_cases.md:19

Unless the no_db option is set to true in the config file...

The code actually uses DatabaseSize::Gb(0) to indicate no database (zaino-state/src/local_cache/non_finalised_state.rs:375-378):

let no_db = match self.config.storage.database.size {
    zaino_common::DatabaseSize::Gb(0) => true,
    zaino_common::DatabaseSize::Gb(_) => false,
};

Originally posted by @gustavovalverde in #807

#854 fixes problems 1 and 2. I'm opening another issue to track problem 3

Originally posted by @nachog00 in #807