Docs: Add comprehensive troubleshooting section to installation guide

[Vasanthadithya-mundrathi]

This PR enhances the installation documentation by adding a comprehensive troubleshooting section.

Changes:

• Added detailed troubleshooting guidance for common installation issues
• Included command examples for diagnosing and fixing problems
• Added verification steps to confirm successful installation

These improvements will help new users overcome common obstacles when getting started with the Agent Development Kit.

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[Vasanthadithya-mundrathi]

Hi maintainers,

Just wanted to mention that this contribution is being made as part of the Google Cloud Multi-Agents Hackathon (https://googlecloudmultiagents.devpost.com/ ).

Could you please trigger the CI/CD checks for this PR by commenting /gcbrun when you have a moment? Thanks!

[eliasecchig]

LGTM, apologies for the delay in merging!