[FRR] Add document for FRR upgrade process

Add a documentation for the FRR upgrade process in SONiC,
using the transition from 10.3 to 10.4.1 as a practical example.
It covers branch creation, patch porting, PR preparation, and testing procedures,
with the goal of helping contributors follow a consistent and reliable upgrade workflow.

Signed-off-by: Yuqing Zhao <[email protected]>

Author: GaladrielZhao