[Docs]: Clarify SSH instructions for launching Appsmith using the AMI

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Documentation Link

https://docs.appsmith.com/getting-started/setup/installation-guides/aws-ami#login-with-root-admin

### Discord/slack/intercom Link

_No response_

### Describe the problem and improvement.

When reviewing the AWS AMI self hosting documentation, it lacks specific instructions on how to SSH into the server:
- how to use the keypair generated for the EC2 instance
- which username to use in the SSH command (some AWS users might assume `ec2-user` if they've only used Amazon Linux)

Additionally, notes on security groups are sparse and may lead users to leave port `22` open to the internet.

I'd suggest the following changes:
- instructions on how to use the SSH keypair generated for Appsmith
- example of the SSH command used (includes using alternate private key, correct username)
- give example of security group rule for SSH access, limited to a single IP address