Fix(Insomnia): update with troubleshooting steps for timeouts only in Insomnia

[juliamrch]

Description

This is a long-lasting issue (+2 years) that I didn't see documented anywhere, and for which Kapa wasn't able to provide a satisfactory answer to one of our users.

I documented the root cause and the troubleshooting steps. Not sure about the choice of the page, lmk if you think another page would be more appropriate.

Closes #4852

Preview Links

https://deploy-preview-4869--kongdeveloper.netlify.app/insomnia/requests/#faqs

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

Documents troubleshooting steps for request “timeouts” that occur only in Insomnia, capturing a long-standing issue and its likely root cause.

Changes:

• Adds a new Troubleshooting subsection under the request timeout preference documentation.
• Describes how an explicitly set Content-Length value can cause failures and provides steps to remove/disable it.

[Guaris]

I would add this to the FAQ section of the same doc. I think a troubleshooting section would be a good addition to this doc if we have more than one type of issue, so like:

## Troubleshooting
This is a list of common troubleshooting issues and their fixes

### Timeouts appear only in Insomnia

### Other issue 

What do you think?

[juliamrch]

I would add this to the FAQ section of the same doc. I think a troubleshooting section would be a good addition to this doc if we have more than one type of issue, so like:

## Troubleshooting
This is a list of common troubleshooting issues and their fixes

### Timeouts appear only in Insomnia

### Other issue 

What do you think?

I considered the option, but I wasn't sure the FAQ would be the right place for troubleshooting this specific issue.