docs(kubernetes_multicluster): update plugin README to match release standard

Rewrites the kubernetes_multicluster plugin README to match the same
standard as the ECS plugin README. Adds How to Build & Run section,
How to Test section, full config reference tables for all piped config
fields, application config fields, stage options, and patch operations.
Removes the stale "only QuickSync is supported" note and updates stage
names to the K8S_MULTI_* prefix introduced in #6880.

Signed-off-by: Mohammed Firdous <mohammedfirdous682@gmail.com>

Author: Firdous231213