docs(amt): missing methods from public API

[LesnyRumcajs]

The current docs page lacks the most useful public API methods (iteration, flushing, creation...) under the private struct AmtImpl. It'd be nice to have them visible.

https://docs.rs/fvm_ipld_amt/0.7.4/fvm_ipld_amt/
https://docs.rs/fvm_ipld_amt/0.7.4/fvm_ipld_amt/type.Amt.html