Docs: Add a troubleshooting note about the kas-address needing a trailing slash

What does this MR do?

This adds an additional Error while dialing failed to WebSocket dial error message to a common configuration typo when the trailing slash is left off the kas-address for a self-managed instance and /-/kubernetes-agent is redirected by nginx to /-/kubernetes-agent/ - the expected handshake response status code 101 but got 301 isn't particularly obvious in that case.

Also changes the markdown code type on another example from plaintext to json

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Ensure that the product tier badge is added to doc's h1.
• Request a review based on the documentation page's metadata and associated Technical Writer.

To avoid having this MR be added to code verification QA issues, don't add these labels: feature, frontend, backend, ~"bug", or database

Review checklist

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• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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