[Doc] Project documentation needs improvement

[jokemanfire]

The doc of the project is currently very incomplete, and we need to improve it

• Add module level document annotations
• Add item level document annotations
• The documentation generated by cargo doc needs to serve as guidance
• Each crate should have its own dedicated readme, with the top-level readme responsible for indexing

[EItanya]

I'll have some time on the plane today, I can have a go at working on this, feel free to assign me.

[jokemanfire]

Very thanks.