docs: fix README Deploy section (Helm not docker), surface in-repo docs

[saadqbal]

The README is mostly good — strong tone, architecture diagram, Security pointer all land — but the Deploy section misleads readers and doesn't surface the rich in-repo documentation.

What's wrong

• Deploy section opens with docker pull tracebloc/client:latest. This repo ships a Helm chart (client/Chart.yaml v1.3.1), so the actual install is helm install.
• External URLs /environment-setup/deployment-overview, /local-linux, /local-macos, /aws don't match the actual paths in the tracebloc/docs tree (local-deployment-guide-linux, local-deployment-guide-macos, eks-client-deployment-guide, azure-deployment-guide).
• README never mentions the in-repo docs/INSTALL.md, docs/MIGRATIONS.md, docs/migration-tools/README.md, client/MIGRATION.md — which are the operational source of truth.
• No mention of the platforms the unified chart supports (AKS / EKS / bare-metal / OpenShift) or the current chart version.
• The NetworkPolicy/CNI prerequisite is in SECURITY.md but easy to miss; ops need to see it before they install.

Acceptance criteria

• Deploy section uses helm repo add + helm install, not docker pull.
• Chart version + platform list called out.
• Table linking to docs/INSTALL.md, docs/SECURITY.md, docs/MIGRATIONS.md, docs/migration-tools/README.md, client/MIGRATION.md.
• External docs URLs fixed to match actual tracebloc/docs paths.
• NetworkPolicy prerequisite surfaced as a callout.
• Existing tone, badge row, architecture diagram, "What the client manages" list, and Security section preserved.

Out of scope (worth filing separately)

• docs/INSTALL.md:203,220,229 still reference tracebloc-helm-charts (stale repo name).
• Empty legacy aks/, bm/, eks/ directories at repo root.
• Verify https://tracebloc.github.io/client actually serves a Helm chart index (GitHub Pages config).