Telegraf Controller: document the machine-readable API spec endpoint with token auth

@sanderson is already working on this.

Since Jun 11, 2026.

## Problem

The new API reference page (#7314, `/telegraf/controller/reference/api/`) points users to the interactive `/api/docs` reference served by their own instance. The page notes that `/api/docs` requires logging in through the browser.

For agents and automation, the human Swagger UI is the wrong target. They need the underlying OpenAPI document, fetched headlessly with a token.

## Proposed change

1. Confirm with the Telegraf Controller team whether the instance serves the raw OpenAPI spec (for example, `/api/docs/openapi.json`) and whether it accepts `Authorization: Bearer tc-apiv1_...` token auth (not only browser session auth).
2. If yes: add a short section to `/telegraf/controller/reference/api/` with a curl example that fetches the spec using an API token. This converts the page's "check your instance" guidance into an introspection instruction an agent can execute.
3. If no (session-auth only or no raw spec endpoint): file product feedback now, while the API is in beta.

## Why now

The docs publish the API's conventions (base path, port, auth schemes) but only one endpoint. The full catalog is only reachable through instance introspection, so making that introspection scriptable is the highest-leverage single addition to the page.

Related: #7320 (llms.txt coverage), #7314 (API reference page)