Improve Variable Outputs documentation page

[Torres-ssf]

The documentation page Variable Outputs, located under the Contracts section could benefit from a more comprehensive explanation and contextual background.

The challenge lies in the fact that this topic is directly related to UTXO and its transaction details, and yet our documentation fails to deliver an explanation about it. Addressing this subject without proper context may lead to confusion for users who are not familiar with UTXO.

An optimal solution would be to first add a dedicated page or section that explains UTXO and its associated transaction details, and only then enhance the content on the Variable Outputs page.

blocked by #449