Improved resource deletion actions/guidance

As a newcomer, after running `ie execute [markdown file] --do-not-delete=false` (the `--do-not-delete` flag could have been skipped as it is false by default), I was expecting that the Azure resources created by the scenario would have been deleted. However, it didn't happen because the markdown I used `scenarios/ocd/CreateLinuxVMSecureWebServer/README.md` doesn't contain the `az group delete --name $MY_RESOURCE_GROUP_NAME` command.

At this point, I would say that the `--do-not-delete` flag applies only if the the scenario contains the `az group delete` command, as confirmed by the comments:

https://github.com/Azure/InnovationEngine/blob/31f6ea5ca65b5ae9fe491c272055464a0773c4f4/internal/engine/execution.go#L37-L38

Therefore, I suggest to change the flag name from `do-not-delete` to `skip-delete-cmds` or `skip-deletion` or something similar. IMHO, that would make the flag usage clearer.