feat: use doc strings from the `abi` in `abigen`

[hal3e]

closes: https://github.com/FuelLabs/fuels-rs/issues/1335

Adds ABI doc strings to contract methods and script's main function.

Checklist

• [x] I have linked to any relevant issues.
• [x] I have added tests that prove my fix is effective or that my feature works.
• [x] I have added necessary labels.
• [x] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [x] I have requested a review from the relevant team or maintainers.