[Documentation] Summary & Readability improvement

[Mathys-Gasnier]

When working on the rust bindings I found it hard to go around the readme, and lost myself often...

This PR tries to make the doc a bit easier to use by adding a summary to make finding information easier. It also increase readability of certain sections by adding separators between functions/macros/structs.

If you think anymore changes to the README can improve readability i'm open to add them too.