docs(readiness): declare account operations, grpc calls, body keys and new companies

[auscaster]

CONTRIBUTING documents three declaration features the readiness engine already reads. Live listings use each of them.

Account operations (north star §5.6). Frantic and Telnyx declare them.

• An account_operations entry declares a sign-up API (registration), or a top-up the agent pays itself (top_up), on an interface's own endpoint. Its spec is the vendor's OpenAPI 3 document.
• Sourcey sends nothing to one. It reads the operation in the spec, following references on the same host.
• A registration endpoint is now the page where a person signs up, requested only to see that it is there.
• A sign-up the spec says needs no credential is an agent registration path. It counts only with an empty requirement, or with none in a document that requires one on its other operations.

A gRPC call and a key in the request body. Modal and Amplitude use these.

• { media_type: application/grpc } declares a gRPC call: a POST to the method's path.
• credential_field names the body field a vendor takes its key in. OpenAPI has no security scheme for a key in the body.

A company new to Sourcey, listed in the same pull request as its declaration.

This folds in #356 and #359, which edit the same file, so the three land in one review. #359's GraphQL wording is left out: #377 already landed it.

[sourcey]

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[sourcey]

A maintainer will review your pull request

Sourcey can't admit this change automatically, so a maintainer reviews it. You don't need to do anything.

Follow your pull request on Sourcey

! A maintainer reviews this change.

• The pull request changes more than the repository's data or one declared pin.