feat(Insomnia): add proxy troubleshooting info for local mock servers

[juliamrch]

Description

Add info on:

• Node.js url server (mockbin doesn't log it at run)
• Proxy interference

Preview Links

https://deploy-preview-4949--kongdeveloper.netlify.app/insomnia/self-hosted-mocks/#create-a-local-self-hosted-mock-server

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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[Copilot]

Pull request overview

Adds troubleshooting guidance to the Insomnia self-hosted mocks reference page to help users run local mock servers successfully, including proxy-related gotchas.

Changes:

• Adds a new “Create a local self-hosted mock server” section with links to Node/Docker/Kubernetes run options.
• Adds an info note warning that Insomnia’s proxy can interfere with local mock traffic and should be disabled while testing.