docs: remove stale Kubernetes manifest reference

[ming1523]

Summary

• remove the stale deployment/fuel-core.yml example from the root README
• clarify that the repository currently ships container build assets under deployment/, but not a checked-in Kubernetes manifest
• point readers to deployment/README.md and the Fuel node operator docs instead

Why

The current README still tells readers to run kubectl create -f deployment/fuel-core.yml, but that file is not present in the repository. Replacing the stale command with pointers to the current deployment assets and operator docs makes the entrypoint more accurate for contributors and operators.

Validation

• verified that deployment/fuel-core.yml does not exist in the current tree
• verified that deployment/README.md exists and is the repository-local deployment entrypoint
• docs-only change, no runtime behavior changed

This is a docs-only change. A no-changelog path seems appropriate if maintainers agree.

[cursor]

PR Summary

Low Risk
Low risk, docs-only change that only updates README guidance and links with no runtime or build impact.

Overview
Clarifies the Docker & Kubernetes README instructions by removing the stale kubectl ... deployment/fuel-core.yml example (the manifest is not checked in) and adding pointers to deployment/README.md plus the official Fuel node operator Kubernetes docs.

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