[Content] Writing contents on StringLiteral and StringRef

[Lorenzobattistela]

I know this is work in progress, and I'm looking forward to contribute with this so we can build a great learning environment on Mojo. That said, I think we can add the builtins for stringliteral and ref (https://docs.modular.com/mojo/MojoBuiltin/StringRef.html) (https://docs.modular.com/mojo/MojoBuiltin/StringLiteral.html). You can assign me for this. Thanks and great work

[mojodojodev]

Great thanks Lorenzo much appreciated