[Alex] ARC-542: Add egress-webhook-service openapi yaml and render it

[AlexWerff]

Note

Adds services/egress-webhook-service/openapi.yaml, surfaces it under a new "Webhook Events" tab, moves webhook security docs to webhooks.mdx, and removes webhook event schemas from the Customer API spec.

• Docs/navigation:
  • Update docs.json to:
    • Replace API Reference with an Overview tab (adds Webhooks page).
    • Add Customer API (OpenAPI services/external-actor-gateway-service/openapi.yaml).
    • Add Webhook Events (OpenAPI services/egress-webhook-service/openapi.yaml).
• API specs:
  • Add services/egress-webhook-service/openapi.yaml defining webhook events: page.version.created|processed|published, form.submitted|enriched|enrichment_failed.
  • Remove embedded webhooks definitions from services/external-actor-gateway-service/openapi.yaml.
• Content:
  • Move webhook signature verification from index.mdx to new webhooks.mdx.

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