feat: document optional healthCheck in connect.yaml templates

[ruiaraujo]

Description and context

Part of the connector HTTP health-check feature. This updates the author-facing starter templates and documentation so connector developers know how to declare an optional HTTP healthCheck in connect.yaml.

The full cross-repo context lives in the connect-services PR (linked in a comment below).

Brief description of the solution

• Added an example healthCheck block to the service and event applications in both the JavaScript and TypeScript connect.yaml templates.
• Documented the healthCheck property and its fields (path, intervalSeconds, timeoutSeconds, unhealthyThreshold) in the README, including in the sample connect.yaml.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How was it tested?

• Manual review of the rendered templates and README; the healthCheck shape matches the schema enforced by the specification package and the deployment pipeline.

Depends on another PR/branch?

• Yes — depends on the schema/pipeline changes in the connect-services PR (linked in a comment below).

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Latest commit: 031efdd

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[ruiaraujo]

Umbrella PR with full cross-repo context: commercetools/connect-services#2047