Improve crates.io and docs.rs documentation

[adizere]

Crate

ibc, relayer, and relayer-cli

Summary

The crates.io and docs.rs presentation of our main crates is minimalistic. Especially for the ibc library, we should improve documentation and presentation.

ibc-proto is better in this regard.

Related: #608 is dependency for this issue

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[romac]

Few other things to look for:

• Some modules don't have any module-level documentation (see first few modules in https://docs.rs/ibc/0.1.0/ibc/index.html)
• Some modules don't export anything (https://docs.rs/ibc/0.1.0/ibc/ics03_connection/handler/conn_open_ack/index.html)