[Synthetics] Update infrastructure monitoring for k8s

[alaudazzi]

Summary

• Add a short "Not for infrastructure or Kubernetes monitoring" note/admonition to solutions/observability/synthetics/index.md. State clearly that the Synthetics app is designed for active synthetic checks of URLs and user journeys, not for host/pod availability monitoring via autodiscovery. Link to the Heartbeat / Elastic Agent alternatives.

• Add a matching section to solutions/observability/synthetics/scale-architect-synthetics-deployment.md (alongside the existing CCS/CCR "do not use" section) documenting the k8s infra monitoring limitation and recommended alternatives.

Note for reviewers

@emilioalvap @eedugon can you confirm whether the recommended alternatives to document are correct:

For infrastructure or {{k8s}} uptime monitoring, use one of the following approaches instead:

• {{heartbeat}} with autodiscovery: Run {{heartbeat}} directly on your infrastructure and use its autodiscovery capabilities to dynamically monitor hosts and pods. Results appear in the {{uptime-app}}.
• {{agent}} with the Uptime Monitors integration: Deploy a standalone {{agent}} and configure the Uptime Monitors ({{heartbeat}}) integration to collect availability data from your infrastructure. Note that the Uptime app is deprecated as of 8.

The Uptime app (/solutions/observability/uptime/) is deprecated as of 8.15 and should not be promoted as the primary destination. I added a note about deprecation.

Closes #1879.

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

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🔍 Preview links for changed docs

• solutions/observability/synthetics/index.md
• solutions/observability/synthetics/scale-architect-synthetics-deployment.md

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✅ Vale Linting Results

No issues found on modified lines!

---

The Vale linter checks documentation changes against the Elastic Docs style guide.

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Docs review summary

Focus areas

• Style and clarity: Vale reported no findings (files were outside the default docs/ scope path). Manual review found minor issues — "Note that" filler and "via" where the style guide prefers "through".
• Jargon: {{heartbeat}}, {{agent}}, {{uptime-app}}, {{k8s}}, {{synthetics-app}} are all substitution variables; no unexplained jargon introduced.
• Frontmatter and applies_to: Frontmatter is unchanged. The new section in scale-architect-synthetics-deployment.md correctly omits a section-level applies_to override (the CCS/CCR section above it has stack: ga because it's stateful-only; the new k8s section applies to both stack and serverless, matching the file-level frontmatter).
• Content type fit: Both additions fit their context — an important admonition in the index page and a parallel "Do not use" section in the scale/architect page, consistent with the existing CCS/CCR section pattern.
• Parent issue satisfaction: Closes #1879. The PR adds the requested admonition to the index and a matching section to the scale/architect doc. Satisfied.

Nits

• Both changed files use via where the style guide prefers through (e.g., "monitoring via autodiscovery" → "monitoring through autodiscovery").
• "Note that" appears in both bullets recommending the Uptime Monitors integration — inline suggestions posted above.

Notes

• The pre-fetched Vale data shows eligible_count: 0 because the default scope is docs-subtree (docs/), and these files live under solutions/. Review was performed manually as this repo's docs span the full repository.
• The PR body already flags the Uptime app deprecation tension: both new sections link to the Uptime app while noting it's deprecated as of 8.15. Authors should confirm with @eedugon / @emilioalvap whether a non-deprecated landing page exists, or whether the link should be dropped entirely from the alternatives list.

Generated by Docs review agent for issue #6367 · ● 301.4K

[eedugon]

LGTM! I've suggested some small changes to make the note shorter, plus fixed some links.
Also the elastic agent suggestion links to autodiscover doc.

[eedugon]

Suggested change

* **{{agent}} with the Uptime Monitors integration**: Deploy a standalone {{agent}} and configure the Uptime Monitors ({{heartbeat}}) integration to collect availability data from your infrastructure. Note that the Uptime app is deprecated as of 8.15.

* **[{{agent}}](/reference/fleet/install-elastic-agents.md) with the Uptime Monitors ({{heartbeat}}) integration**: Deploy {{agent}} and configure the Uptime Monitors integration to collect infrastructure availability data. For [Kubernetes autodiscovery](/reference/fleet/elastic-agent-kubernetes-autodiscovery.md), hints-based autodiscovery requires standalone mode. Note that the Uptime app is deprecated as of 8.15.

[eedugon]

Suggested change

* **{{agent}} with the Uptime Monitors integration**: Deploy a standalone {{agent}} and configure the Uptime Monitors ({{heartbeat}}) integration to collect availability data from your infrastructure. Note that the Uptime app is deprecated as of 8.15.

* **[{{agent}}](/reference/fleet/install-elastic-agents.md) with the Uptime Monitors ({{heartbeat}}) integration**: Deploy {{agent}} and configure the Uptime Monitors integration to collect infrastructure availability data. For [Kubernetes autodiscovery](/reference/fleet/elastic-agent-kubernetes-autodiscovery.md), hints-based autodiscovery requires standalone mode. Note that the Uptime app is deprecated as of 8.15.