docs: Write a "effective-go-krm-function" guide

[yuwenma]

We need to better explain how to write KRM functions, with proper examples.

• [WIP] Function SDK guide #3367 effective Go KRM function guide
• Improve functionRunner interface krm-functions-sdk#599 Example on how to use the Runner Interface
• feat: add example for effective go krm function krm-functions-sdk#602 a simple scaffolding sample to
  let users get started.

[tuananh]

Love this. I was looking for this the other day :)