docs(scheduler): add scheduler user guide for v1.7.x #974

[zhongwen666]

Changes

• Add comprehensive scheduler user guide for version-1.7.x docs in English and Simplified Chinese (zh-Hans)
• Introduce ROCK_FORCE_PRIMARY_POD env var so the scheduler can run on a single node for local dev / testing
• Expand rock-conf/rock-local.yml with a full scheduler configuration example (image cleanup, file cleanup, container cleanup, image pull)

Code Changes

• docs/versioned_docs/version-1.7.x/User Guides/scheduler.md: new English scheduler guide (working principle, configuration, built-in tasks, custom task development, observability)
• docs/i18n/zh-Hans/docusaurus-plugin-content-docs/version-1.7.x/User Guides/scheduler.md: Simplified Chinese counterpart
• rock/env_vars.py: declare ROCK_FORCE_PRIMARY_POD (default false) with lazy os.getenv lookup
• rock/utils/system.py: is_primary_pod() short-circuits to True when ROCK_FORCE_PRIMARY_POD is set
• rock-conf/rock-local.yml: add scheduler block with built-in task class examples and parameters

Related Issue

Closes #974