ReadMe Personalized Docs Webhook

ReadMe's Personalized Docs Webhook is the outbound, customer-hosted webhook the developer hub calls at user sign-in to look up a reader's account data. ReadMe POSTs a JSON body containing the user's email, signs the request with a Stripe-style `ReadMe-Signature` header (`t=,v0=`) using the project's webhook secret, and expects a JSON response describing the user (apiKey, keys array, server-variable parameters, Basic Auth, GraphQL bearer, etc.) which the hub then merges into the rendered API Reference, Try-It console, and Recipes. The single documented event is the user-data lookup.