docs: Troubleshooting section for IPv6 policy

[alanty]

What does this PR do?

Adds a section in the blueprints troubleshooting describing how to create the IPv6 policy for EKS.

Motivation

The spark IPv6 blueprint fails to deploy if the AmazonEKS_CNI_IPv6_Policy doesn't exist to attach to IAM roles with the error below:

Error: attaching IAM Policy (arn:aws:iam::1234567890:policy/AmazonEKS_CNI_IPv6_Policy) to IAM Role (core-node-group-eks-node-group-20241111182906854800000003): operation error IAM: AttachRolePolicy, https response error StatusCode: 404, RequestID: 9c99395a-ce3d-4a05-b119-538470a3a9f7, NoSuchEntity: Policy arn:aws:iam::1234567890:policy/AmazonEKS_CNI_IPv6_Policy does not exist or is not attachable.

More

• Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
• Mandatory for new blueprints. Yes, I have added a example to support my blueprint PR
• Mandatory for new blueprints. Yes, I have updated the website/docs or website/blog section for this feature
• Yes, I ran pre-commit run -a with this PR. Link for installing pre-commit locally

For Moderators

• E2E Test successfully complete before merge?

Additional Notes