High level design doc of Patu CNI

[vishnoianil]

Describe the bug
Add a high level design doc of the Patu CNI, that describe the overall design and how individual feature works. This document should contain very high level information to give a 20000 feet view of the design. Individual features and their details will be documented in respective separate document.

To Reproduce
No document exist as of now in the repo.

Expected behavior
A document explaining the high level design, preferably with lots of pictures :).