[stress testing] Add docs for recommended directory structure and helmignore

There are some downsides to a fully flat structure: helm includes all files in the directory by default and this can result in issues such as the chart being too large. The [layout docs](https://github.com/Azure/azure-sdk-tools/blob/main/tools/stress-cluster/chaos/README.md#layout) should include guidance on where to keep source code and other non-helm assets (e.g. in a sub-directory) and add a quick description/guide for the `.helmignore` file.

Also we should update the troubleshooting guide with the "chart too big" error message.