Document that immutable folder discovery is recursive

[jpraynaud]

Why

The immutable folder discovery is recursive in all the Mithril nodes, but this behavior is not well documented or tested.

What

Document and test the recursive folder discovery behavior in the Mithril nodes.

How

• Document the function with the recursive behavior in the mithril-cardano-node-internal-database
• Test the recursive behavior of the discovery in mithril-cardano-node-internal-database
• Document the command options using the recursive behavior:
  • in the client
  • in the signer
  • in the aggregator