pw_transfer: Start docs and proto

- Create the pw_transfer module.
- Start the pw_transfer docs.

Change-Id: I2d3b4f3e719960167ee28ba4ca84eb809bf94bc1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49022
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Keir Mierle <keir@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>