Breaks out the Hello World example to create a new
webhook-walkthrough.md. Includes minor edits so the two docs
read well. Adds the new page, "Webhook walkthrough", to the TOC.
Fixes#3498
This directory contains scripts that are used in building, managing,
testing, and other forms of work in a Zulip development environment.
Note that tools that are also useful in production belong in
scripts/ or should be Django management commands.