docs(automation): document workflow activity and webhooks

[halfofning]

Summary

• Regenerate the public control-plane OpenAPI artifact for workflow webhooks.
• Document Workflows sections and top-level Activity.
• Remove AcornOps Events guidance and update workflow/issue navigation instructions.

Cross-repo change set

Tracking issue: none.

Related PRs:

• acornops/control-plane#23
• acornops/management-console#23
• acornops/docs-website#15

Merge order:

1. control-plane
2. management-console
3. docs-website

Compatibility: intentional greenfield breaking cleanup; no legacy event-trigger API aliases are retained. Console legacy Runs and Triggers URLs redirect to Activity or the matching Workflows section.

Validation

• npm run check: passed across 41 pages.
• npm run validate: passed.
• npm run links: passed with no broken links.
• git diff --check: passed.

Docs impact

• Updates the public OpenAPI reference and operator/user guidance for workflows, Activity, schedules, and incoming webhooks.

Residual risk

• Merge after the control-plane producer PR so published documentation matches the deployed API contract.