Kestra internal storage default configuration

[loicmathieu]

Issue description

On the documentation, it is written that the default internal storage configuration uses the local implementation, see https://kestra.io/docs/administrator-guide/configuration/storage

But it's not true, there is no default for the storage which complicates a first setup if our docker compose is not used.

Two things can be done:

• Set the default storage, but as it's not a good default for Kubernetes it may not be a good idea
• Enhance the documentation to say that a storage must be configured, the installation guide must also be updated.