Updating technique references for Unit42 VM Attack Path article.

[siigil]

What does this PR do?

Adds references to https://unit42.paloaltonetworks.com/cloud-virtual-machine-attack-vectors/ for applicable techniques mentioned in this writeup:

Cloud	Writeup Technique	Writeup Name	Stratus Technique
AWS	Direct Code Execution	Use SSM Run Command to Execute Code	/AWS/aws.execution.ssm-send-command.md
AWS	SSH Key Push	Use EC2 Instance Connect to Push SSH Keys	/AWS/aws.lateral-movement.ec2-instance-connect.md
AWS	Startup Script Manipulation	Modify Startup Scripts in User Data	/AWS/aws.execution.ec2-user-data.md
AWS	SSH Over Middleware	Use SSM Session Manager to Log into a VM	/AWS/aws.execution.ssm-start-session.md
Azure	Direct Code Execution	Use Virtual Machine Run Command to Execute Code	/azure/azure.execution.vm-run-command.md
Azure	Direct Code Execution	Use a Custom Script Extension to Run Scripts	/azure/azure.execution.vm-custom-script-extension.md

Motivation

Adding references to Unit42 VM techniques article in support of #527.

Checklist

N/A as documentation update.

• [ ] The attack technique emulates a single attack step, not a full attack chain
• [ ] We have factual evidence & references that the attack technique was used by real malware, pentesters, or attackers
• [ ] The attack technique makes no assumption about the state of the environment prior to warming it up