[DOC] Add example code to generate hash

[Hugoo]

generate the hash of the uploaded

https://docs.lukso.tech/learn/assets#generate-the-json-file

Add more example code to show how it is done, esp. the part the hashes the files.

It could be nice to make a reference to:

https://docs.lukso.tech/tools/lsp-utils/LSP2ERC725YJSONSchema#encodeasseturl