Extend nested struct recipe to show how to serialise the nested struct data and pretty print using serde_json for debugging

[ltfschoen]

I'd like to suggest expanding on this https://substrate.dev/recipes/structs.html#nested-structs, by showing how someone would use stringify the nested struct using serde and pretty print it, similar to how i tried to do it here https://matrix.to/#/!HzySYSaIhtyWrwiwEV:matrix.org/$161545785896192eWpjV:matrix.org?via=matrix.parity.io&via=matrix.org&via=corepaper.org