[Internal]: Document Heartbeat/Agent autodiscovered monitors in the Synthetics app (read-only)

### Description

Document that **Heartbeat / Elastic Agent–run monitors with no Synthetics saved object** (most notably Kubernetes/Docker **autodiscovery** monitors) now appear in the **Synthetics app** as **read-only** entries, with clear provenance (“Managed by Heartbeat / Elastic Agent”).

**What:** Users can see status and config for these externally managed monitors in the Synthetics overview (and eventually detail views). They cannot edit or delete them from the Synthetics UI.

**When:** Targeted for Elastic Stack **9.6**.

**Why:** The Uptime app is deprecated; customers migrating off Heartbeat previously lost visibility of autodiscovered monitors even though the data is still ingested. This restores read-only visibility in Synthetics (management remains outside the UI, per existing architecture guidance).

#### Suggested doc coverage

- Note that ping-derived Heartbeat/Agent monitors without a Synthetics saved object appear as read-only in Synthetics (similar to CCS/remote monitors).
- Call out the **index requirement**: monitors must write to `synthetics-*` to appear.
- **Elastic Agent–managed synthetics** (Fleet / k8s / Docker autodiscovery via the synthetics integration): appear automatically.
- **Standalone Heartbeat `browser`**: appear automatically.
- **Standalone Heartbeat `http` / `tcp` / `icmp`**: need a `data_stream` block so events route to `synthetics-*` (default is still `heartbeat-*`).
- Clarify this is **visibility only** — not management from the Synthetics UI (align with [Scale and architect a Synthetics deployment](https://www.elastic.co/docs/solutions/observability/synthetics/scale-architect-synthetics-deployment)).

### Resources

- Product issue: https://github.com/elastic/kibana/issues/273494
- Implementation PR: https://github.com/elastic/kibana/pull/274947
- Related enhancement: https://github.com/elastic/enhancements/issues/28500

### Which deployment methods does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

Feature is always-on in the Synthetics app wherever that app is available. Primary customer value is for Agent/k8s autodiscovery and Heartbeat migrations (most common on self-managed / ECK). Classic standalone Heartbeat writing only to `heartbeat-*` remains out of scope until routed to `synthetics-*`.

### What Elastic Stack release is this request related to?

9.6

### Serverless release

N/A / with Stack promotion — confirm whether Serverless needs a separate callout (Agent-managed synthetics may apply; classic Heartbeat does not).

### Collaboration model

The documentation team will create the first draft

### Point of contact.

**Main contact:** @shahzad31

**Stakeholders:** @elastic/actionable-obs-team

Start by reading the linked “Scale and architect a Synthetics deployment” guidance and the suggested coverage in this issue. Document read-only Heartbeat and Elastic Agent monitor visibility, the synthetics-* index requirement, supported deployment paths, and the fact that management remains outside the Synthetics UI; include the 9.6 targeting and confirm whether Serverless needs a separate callout.

Written by the indexing model from the issue text.