Integrations marketplace and connect guides

The Integrations index is a browsable marketplace with search, category filters, and three tiers: built-in destinations, automation platforms, and popular apps reached through Zapier. Each app has its own detail page, and the connect guides for webhooks, the REST API, Make, and n8n all share one create-a-token-and-copy flow. The Custom Webhooks screen uses cards that read the same at every width. Failure reasons are written in plain English, so a webhook reads "Your endpoint returned 503" or "Couldn't reach your endpoint" rather than a bare status code.