Set up API documentation from source code

[mondeja]

Currently the API is documented in the README but is not complete. This is not maintainable because is not generated from source code.

Instead, we should document it using docstrings in functions and arguments and provide a nice introduction in https://docs.rs/leptos_hotkeys/0.2.0-alpha.1 taken from part of the README using cargo-readme or something like that. Rust developers are more accustomed to reading the documentation on docs.rs.