dawn-cmake/docs/buffer_mapping.md

• Buffer mapping dawn wire memory transfer interface design
  • https://docs.google.com/document/d/1JOhCpmJ_JyNZJtX6MVbSgxjtG1TdKORdeOGYtSfVYjk/edit?usp=sharing&resourcekey=0-1bFi47mR1jkBLdRFxcTVig
• TODO: make a md doc targeted at code walkthrough for contributors