Add note about how YAML is picky about indentation

[kaganjd]

Places where this could live:

• https://ngrok.com/docs/agent/config/v3/
• https://ngrok.com/docs/traffic-policy/getting-started/agent-endpoints/cli/

Or could be its own guide like this:

• https://www.elastic.co/guide/en/beats/heartbeat/current/yaml-tips.html