[Multi_K8s-Plugin] Update plugin README to release standard

[mohammedfirdouss]

What this PR does:

Updates the kubernetes_multicluster plugin README to match the same standard as other plugin README.

• Adds How to Build & Run section with Makefile commands, --force-plugin-redownload note, and local kind cluster setup steps
• Adds How to Test section explaining how to run unit tests and the role of envtest
• Adds Directory Structure overview
• Adds full Config Reference tables for all fields: piped config, deploy target config, application config, all 8 stages, Helm/Kustomize options, traffic routing (PodSelector + Istio), and patch operations
• Removes the stale "only QuickSync is supported" note
• Removes the "url is a dummy" comment (no longer accurate)
• Updates all stage names to the K8S_MULTI_* prefix introduced in [Multi_K8s-Plugin] rename stages to avoid conflicts with kubernetes plugin #6880

Why we need it:

The plugin README was a rough developer guide written during the mentorship term. Before a binary release can be triggered (via the plugin_release workflow), the README should be accurate and useful for contributors and users — matching the standard set by the ECS plugin.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

• How are users affected by this change: Documentation only — no code changes.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[mohammedfirdouss]

@rahulshendre @eeshaanSA @khanhtc1202 please review when free