[cli] Hint at troubleshooting docs on timeout

[ShadiBahaa]

Summary

When multipass launch times out waiting for an instance to become ready (e.g. due to a firewall blocking DHCP, or network misconfiguration), the error message provides no guidance on what to try next.

This adds a link to the troubleshooting documentation when the launch failure indicates a timeout, giving users a starting point to diagnose the issue.

Before:

launch failed: The following errors occurred:
test-vm: timed out waiting for response

After:

launch failed: The following errors occurred:
test-vm: timed out waiting for response
For troubleshooting, see https://documentation.ubuntu.com/multipass/stable/how-to-guides/troubleshoot/

Based on reviewer feedback from #4500:

• Does not mention any specific cause (multiple causes are possible)
• Points to the general troubleshooting page rather than a specific sub-page
• Uses a single string construction

Test plan

• Verify that launch timeouts now include the troubleshooting link in the error output
• Verify that non-timeout launch failures (invalid disk size, invalid hostname, etc.) are unaffected
• Verify that the INVALID_NETWORK error still shows its own troubleshooting link

Closes #533