docs: add manual remediation Dag example with Params and dry-run guard

[vividbaek]

Add a small documentation section to params.rst showing how to combine
Params, dry-run defaults, max_active_runs=1, and task-level guards
(pools, execution timeouts) in a manually triggered remediation Dag.

This pattern is common in operational workflows such as backfill,
reprocessing, or data repair where incorrect user input can directly
affect external systems. The example demonstrates:

• Param schema validation for user input
• Runtime validation inside a task (e.g., date range checks)
• dry_run=True as a safe default for first-time triggers
• max_active_runs=1 to prevent concurrent runs of the same Dag
• Pool and execution timeout to protect the external system

This is a documentation-only change. No runtime behavior is affected.

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Was generative AI tooling used to co-author this PR?

• Yes — GLM 5.1

Generated-by: GLM 5.1 following the guidelines