GitLab

Add some documentation on how extension APIs work, and best practices
for writing a new one.

Bug: None
Change-Id: I82e2ca785d474ba533dec244b91e36b9b55178b2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2024122Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736129}