Telegraf Controller: define canonical sources for duplicated authentication content

## Current state

On the `telegraf-controller-ga` branch, the following content appears in more than one page:

1. **Authentication providers table** (Local, LDAP, OIDC with availability and sign-in method):
- `content/telegraf/controller/authentication/_index.md`
- `content/telegraf/controller/reference/authentication-authorization.md`

The two copies have already diverged: the Local provider's availability is "Free" in `authentication/_index.md` and "Free tier" in the reference page.

2. **API token facts** (`tc-apiv1_` prefix, value shown once at creation, permissions bounded by the owning user's role):
- `content/telegraf/controller/tokens/_index.md`
- `content/telegraf/controller/reference/authentication-authorization.md`

3. **`Authorization` header example** (`Bearer`/`Token` scheme with a `tc-apiv1_` token):
- `content/telegraf/controller/tokens/use.md`
- `content/telegraf/controller/reference/api.md` (#7314)

## Recommended changes

For each item, designate one page as the canonical source and update the other pages to either link to it or transclude it:

- Decide whether the task page or the reference page owns each fact.
- Replace non-canonical copies with a link, or extract the content into a shared file per the [shared content guidance](https://github.com/influxdata/docs-v2/blob/master/DOCS-CONTRIBUTING.md#shared-content).
- Reconcile the "Free" / "Free tier" divergence in the providers table as part of the change.

## Related

- #7314 (API reference page)
- #7277 (authentication providers documentation)
- #7324 (cross-links between the authentication and tokens sections)

Read the shared content guidance in DOCS-CONTRIBUTING.md, then compare the duplicated content in the listed authentication, tokens, and reference pages. Review related issues #7314, #7277, and #7324 before choosing canonical sources. Done means the duplicated facts have one maintained source, the other pages link to or transclude it, and the "Free"/"Free tier" wording is reconciled.

Written by the indexing model from the issue text.