Remote clusters: ECE -> ECK referencing steps of a procedure of another doc

Affected document: https://www.elastic.co/docs/deploy-manage/remote-clusters/ece-enable-ccs-for-eck

This approach is not ideal:

The equivalent doc of [Remote Clusters from ECH to ECK ](https://www.elastic.co/docs/deploy-manage/remote-clusters/ec-enable-ccs-for-eck) doesn't face this issue as we recently updated it.

We should write a clear and valid procedure instead of linking to N steps of another do that could eventually change.

Consider using snippets if the procedures of both pages are really equivalent or similar.

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## Elastic Docs AI Scoping 🤖

Docs issue scope

### Summary
The issue asks to fix `ece-enable-ccs-for-eck.md`, which currently cross-references steps from a different document (`ece-remote-cluster-self-managed.md`) in its TLS certificate procedure, rather than providing a self-contained how-to. There are no linked code PRs because this is a pure documentation quality improvement; the scope was confirmed directly by reading the source files. The equivalent ECH→ECK page (`ec-enable-ccs-for-eck.md`) was already fixed and serves as the model.

### Request accuracy
Accurate — the file at `deploy-manage/remote-clusters/ece-enable-ccs-for-eck.md` (line 102, TLS certificate tab) tells users to "Follow the steps provided in [Access clusters of a self-managed environment](ece-remote-cluster-self-managed.md), and specifically the first three steps..." instead of providing those steps inline.

### Next action for author
Rewrite the "Establish trust in the ECE cluster" section in the TLS certificate (deprecated) tab of `ece-enable-ccs-for-eck.md` to be self-contained, mirroring the structure of `ec-enable-ccs-for-eck.md`.

### Impact: High

### Scope boundary
The API key tab and all non-TLS-certificate sections of this page do not need changes; the `ece-remote-cluster-self-managed.md` page itself does not need changes.

### Recommended documentation targets

| Page | URL | Action | Impact | Confidence | Why this page? |
|------|-----|--------|--------|------------|----------------|
| Connect ECE deployments to ECK clusters | https://www.elastic.co/docs/deploy-manage/remote-clusters/ece-enable-ccs-for-eck | Update existing page | High | High | This is the broken page — the cross-reference in the TLS tab needs to be replaced with a self-contained procedure. |

### Recommendations

1. **Rewrite** the "Establish trust in the ECE cluster" section in the TLS certificate (deprecated) tab of `ece-enable-ccs-for-eck.md`. Replace the sentence that defers to `ece-remote-cluster-self-managed.md` with numbered sub-steps equivalent to those in the ECH page (`ec-enable-ccs-for-eck.md`, lines 95–122), adapted for the ECE UI (e.g., ECE Security page path, correct Scope ID `default.es.local`, ECE trust settings UI).

2. **Consider adding** a "Update the downloaded `trust.yml` file for ECK compatibility" section (as exists in `ec-enable-ccs-for-eck.md`, lines 124–162) if the same `trust.yml` subject-name correction is required for ECE-issued certificates.

3. **Evaluate snippets** — if the ECE and ECH procedures for the ECK-side trust configuration (kubectl steps, YAML spec) are identical, extract them into a new shared snippet in `_snippets/` and include it from both pages. The page already uses this pattern extensively.

### Notes

- The page is a how-to; all additions should stay in that form (numbered steps, concrete commands, no conceptual digressions).
- The TLS certificate tab is already marked `(deprecated)` — keep that label and limit changes to accuracy; no need to expand this tab's scope.
- Follow-on assembly work is minimal: no new pages, redirects, or nav changes needed.

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