docs: add AWS-supported Cilium management guide

[parlakisik]

Add comprehensive documentation for managing AWS-supported Cilium in EKS Anywhere clusters. This guide covers:

• Overview of EKS-A managed vs self-managed Cilium approaches
• Installing AWS-supported Cilium with Helm for self-management
• Pros and cons of self-managing Cilium deployment
• Migration path from open source to AWS-supported version
• Transitioning from self-managed to EKS-A managed Cilium
• Verification steps and expected outputs

The documentation helps users understand their options for managing Cilium CNI and provides clear instructions for each approach.

Issue #, if available:
https://github.com/aws/eks-anywhere-internal/issues/3664

[parlakisik]

/approve

[eks-distro-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: parlakisik

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.94%. Comparing base (d864ad5) to head (d01fef7).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10256      +/-   ##
==========================================
- Coverage   69.96%   69.94%   -0.02%     
==========================================
  Files         681      681              
  Lines       41825    41802      -23     
==========================================
- Hits        29263    29240      -23     
  Misses      10764    10764              
  Partials     1798     1798              

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[echapman13]

Can they only implement this if they're upgrading EKS-A versions? What if they want to offload ownership while staying on the same EKS Anyhwere version?

[parlakisik]

if they want to offload ownership , they can use first option and use aws supported cilium.
if customer is using cilium oss , they can upgrade to aws supported cilium .
if customer is using different cni (flannel , calico , etc ), They they must remove it and install aws supported cilium

[echapman13]

I see. So the process is more complex if you're using a different CNI entirely?

[parlakisik]

That's correct . We are not supporting cni replacement if customer is using different CNIs