Setup: Configuring EventGrid namespace properties fails

### This issue is for a: (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [ ] feature request
- [X] documentation issue or request
- [ ] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
The Setup section asks the user to create/configure an EventGrid namespace. An Azure CLI command is given to set properties:
```
az resource create --id $res_id --is-full-object --properties '{
"properties": {
"isZoneRedundant": true,
"topicsConfiguration": {
"inputSchema": "CloudEventSchemaV1_0"
},
"topicSpacesConfiguration": {
"state": "Enabled"
}
},
"location": "westus2"
}'
```
However, in the current version of EventGrid there is an MQTT option, which when enabled, will set these properties to the desired value (see pic):

Without enabling the MQTT option these properties are read-only and the command will fail when trying to set them.

### Expected/desired behavior
The documentation should be updated to instruct the user to enable the "MQTT" feature.

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