Document how to debug our webapp boilerplate

In !443 (comment 803927) we had a conversation about attaching the VS Code debugger to the webapp process and it was made apparent that this is woefully underdocumented.

Once uis/devops/webapp-boilerplate#106 (closed) is closed, add a "how to attach a debugger to our webapp" page into the guidebook. Include a launch.json configuration suitable for VS Code.

Thanks to @hwtb2 for raising this.