Update the `vercel` README with accurate information about the managed Vercel endpoint

[vishaangelova]

Currently, the README of the vercel ("Vercel OpenTelemetry Integration”) integration is inaccurate with regard to where the user can find the Vercel managed endpoint.

The endpoint seems to be available in the UI only for Observability Serverless projects at the moment (in the staging environment, but not yet in production). It is not surfaced on the UI for ECH deployments. The integration docs refer to the endpoint being available for both Serverless and ECH (cf. Requirements).

Step 2: Gather the managed endpoint URL and API key from Elastic section refers users to look for the endpoint on the project/deployment’s Manage page. This is inaccurate, as currently the Vercel endpoint is only surfaced on the Add data page of a Serverless Observability project, after clicking More in the Connect directly to the endpoint section.

If users can use the endpoint on ECH deployment by forming it from the OpenTelemetry/managed OTLP endpoint (the ingest endpoint), then this should also be explicitly explained, including how to form the Vercel endpoint from the base ingest endpoint.