Update docs.rs

[ra0x3]

• docs.rs is used as a reference to see/understand how crate APIs work
• Currently, the Fuel indexer's docs.rs is not in a great spot
  • The build fails on release https://docs.rs/crate/fuel-indexer/0.1.8/builds
• We should get our docs.rs up-to-date so devs can read more about our APIs

Additional context

• Here are fuel-core's docs.rs docs https://docs.rs/fuel-core/0.14.1/fuel_core/
• Here are the fuel-indexer's docs.rs docs https://docs.rs/crate/fuel-indexer/latest
• This will require writing good docstrings going forward as well