Publish backend docs to docs.rs

[jbesraa]

No description provided.

[kokal33]

cargo doc --open <== To build docs locally and open them

[kokal33]

@jbesraa I have noticed that there is already a package called fp-core in the rust docs. Its a rust functional core package, and our project is also named fp-core. Just an observation, let's see what will happen when we push it to cargo.
https://docs.rs/releases/search?query=fp-core

It is ready to be pushed so the docs show in docs.rs
#12