docs: add Google Colab setup and troubleshooting section (#2025)

### Summary
Add a Colab-specific setup and troubleshooting section to README to help
users run MONAI tutorials reliably on Google Colab.

### Changes
- Document explicit installation of PyTorch 2.6.0 (CUDA 12.4) and MONAI
1.5 on Colab
- Add troubleshooting notes (torchaudio mismatch, filelock conflict,
num_workers warning)
- Include a quick smoke test command to verify the setup

### Motivation
Many users start with Colab and often hit dependency conflicts. This
section improves first-time success and reproducibility.

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Signed-off-by: minsu <minsu.kim08@gmail.com>
Signed-off-by: Minsu Kim <60283244+minsuking@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Eric Kerfoot <17726042+ericspod@users.noreply.github.com>