Add getting started guide for rust-libp2p

[salmad3]

Done Criteria

TO DO

Why Important

For a long time, this content has only referenced the rust-libp2p examples directory.

[mxinden]

For the reasons outlined by @Menduist in #245 (comment) I don't think we should move the Rust examples here, unless we can enforce in an automated way that they will never be outdated. I think setting up such automation is worth the benefit. In other words, I suggest keeping the Rust example code in the rust-libp2p repository.

[salmad3]

The proposal is to have at least one getting started-like guide similar to Go and JS. We can continue to refer to the examples in the rust-libp2p repo.