Move inline YAML doc examples to real manifest files

[negz]

Description of your changes

Several docs pages inlined full, standalone manifests directly in the markdown instead of pulling them from docs/manifests/ via the manifests shortcode. The rest of the docs use this pattern so that examples are validated against the generated schemas (docs/utils/validate/validate_manifests.py). The external-provider walkthrough in model-endpoint.md also duplicated its ModelEndpoint under a different name in a separate "## Example" section.

This moves each inline example to a file and points the page at it:

• The external-provider ModelEndpoint and its ModelService pairing in model-endpoint.md, deduping the two ModelEndpoint copies into one file (concepts/model-endpoint.yaml) and dropping the now-redundant "## Example" section.
• The landing page's ModelDeployment/ModelService in overview/_index.md.
• The PodMonitor in collecting-engine-metrics.md, whose sibling manifests on that page already used the shortcode. Its follow-up snippet had a stale kubectl apply -f podmonitor.yaml referencing a file the reader never created; it now only covers the port-forward step, since the shortcode surfaces the real apply command.

The partial spec: fragments elsewhere in the docs (e.g. in model-service.md, model-deployment.md, model-cache.md) are left inline, since they're deliberately illustrating one field at a time and aren't full, applyable manifests.

I have:

• Read and followed Modelplane's contribution process.
• Run nix flake check (or ./nix.sh flake check) and made sure it passes.
• Added or updated tests covering any composition function changes.
• Signed off every commit with git commit -s.

[Copilot]

Pull request overview

Moves standalone YAML examples out of inline Markdown fences into docs/manifests/ files and switches the affected docs pages to use the manifests shortcode, so examples are centralized and schema-validated consistently across the docs.

Changes:

• Added new manifest files for the overview page’s ModelDeployment/ModelService, and for the engine-metrics PodMonitor example.
• Deduplicated the external-provider walkthrough by moving the ModelEndpoint + paired ModelService into shared concept manifests and removing the redundant “Example” section.
• Updated the impacted docs pages to reference the new manifest files via {{< manifests ... >}}.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/manifests/overview/model-service.yaml	New ModelService manifest used by the docs landing/overview page.
docs/manifests/overview/model-deployment.yaml	New ModelDeployment manifest used by the docs landing/overview page.
docs/manifests/examples/collecting-engine-metrics/podmonitor.yaml	New PodMonitor manifest extracted from the inline metrics-scraping docs example.
docs/manifests/concepts/model-service-external.yaml	New concept manifest for routing a ModelService to both in-cluster replicas and an external ModelEndpoint.
docs/manifests/concepts/model-endpoint.yaml	Updated concept manifest for creating an external-provider ModelEndpoint.
docs/content/overview/_index.md	Replaced inline YAML examples with manifests shortcode references.
docs/content/models/model-endpoint.md	Replaced inline YAML with manifest references; removed duplicated example section.
docs/content/examples/collecting-engine-metrics.md	Replaced inline PodMonitor YAML with a manifest reference and adjusted the follow-up command snippet.

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