cfn-guard-lambda: document how to pass YAML templates and multi-line Guard rules

**Describe the issue**
Document how to pass YAML templates and multi-line Guard rules as inputs to `cfn-guard-lambda`. For example, given a sample YAML template such as:

```
---
AWSTemplateFormatVersion: '2010-09-09'
Description: Sample template
Resources:
SampleVolume:
Type: AWS::EC2::Volume
Properties:
Encrypted: true
Size: 10
```

and a sample Guard rule:

```
AWS::EC2::Volume {
Properties {
Encrypted == true
Size <= 10
}
}
```

document and provide an example of the expected format to use to pass information (as in above examples) as part of the [payload structure](https://github.com/aws-cloudformation/cloudformation-guard/tree/main/guard-lambda#calling-the-aws-lambda-function).

**Any examples**
-

**Operating System:**
-

**OS Version**
-

**Additional context**
-

Start with the guard-lambda documentation at the linked “Calling the AWS Lambda Function” payload structure. Add YAML template and multi-line Guard rule examples showing the expected input format, then verify that the documented payload matches the examples and can be followed by a caller.

Written by the indexing model from the issue text.