docs: add koord-scheduler deployment modes

[officialasishkumar]

Summary

• add a koord-scheduler deployment modes user manual covering standalone scheduler deployments, HA replicas, and multi-scheduler/multi-master sharding
• document which scheduler plugins should be removed for scheduler-only installs and how to route workloads to scheduler shards
• add the page to the User Manuals task scheduling sidebar

Addresses koordinator-sh/koordinator#2354.
Addresses koordinator-sh/koordinator#2758.

Verification

• helm template koordinator charts/koordinator --namespace koordinator-system with the documented --show-only scheduler/CRD template list
• yarn install --frozen-lockfile
• npm run build

[koordinator-bot]

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