Add documentation for canceling late flow runs

[zzstoatzz]

closes #19313

this PR adds a how-to guide for automatically canceling late flow runs using Prefect automations. when a work pool is paused or workers go offline, scheduled runs can accumulate in a Late state. this guide shows how to prevent queue buildup by using an automation that cancels runs when they become late.

what's included

• new how-to guide at docs/v3/how-to-guides/automations/cancel-late-runs.mdx
• updated docs/docs.json to include the new page in navigation
• demonstration script in sandbox/cancel_late_runs.py (not committed, used for verification)

the guide covers

• creating a deployment
• creating the automation via YAML + CLI
• targeting specific deployments using deployment name in match_related
• explanation of when runs become late
• what happens when runs are cancelled

testing

verified the automation works end-to-end:

• created test deployment and automation
• created flow runs that became late (no worker to pick them up)
• confirmed automation cancelled all late runs
• tested both global (all runs) and targeted (specific deployment) configurations

🤖 Generated with Claude Code