Add proper documentation

[pepperoni21]

Currently the only documentation we have is through the README.md and the examples folder (which do not cover everything btw).

We should add some proper documentation that illustrates each binding/feature of the lib inside a docs folder.

I don't really have time to do that currently so if someone's down to do it let me know!

[Madoshakalaka]

Some items are missing on docs.rs, like the send_function_call function, the FunctionCallRequest struct etc. I think some feature flags were missing during the doc generation?

[boydjohnson]

This explains how to include optional features in docs.rs builds. I will have a PR up soon that includes all features in the docs.rs builds.

https://users.rust-lang.org/t/how-to-document-optional-features-in-api-docs/64577

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See #84