Add code overview documentation

[Pana]

To ease the process of community developer involve into the developing of conflux-core, A code overview doc should been provided like libra, and provide doc for core components.

And the core code organization should follow crate workspace, consider this change will be very big, it will be a long work to finish, maybe crate by crate.

[Thegaram]

We actually have some of this in our old documentation (here). What's left to do is to write a design doc for the missing components and migrate this content to our new docs.