Split example gallery into subsections

[NicolasHug]

It will make it easier to refer to the transforms v2 examples. It looks like this now:

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/7849

• 📄 Preview Python docs built from this PR

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❌ 6 New Failures, 1 Unrelated Failure

As of commit 1f9748d with merge base 6241d47 ():

NEW FAILURES - The following jobs have failed:

• bc (gh)
• unittests-windows (3.10, windows.4xlarge, cpu) / windows-job (gh)
• unittests-windows (3.11, windows.4xlarge, cpu) / windows-job (gh)
• unittests-windows (3.8, windows.4xlarge, cpu) / windows-job (gh)
• unittests-windows (3.9, windows.4xlarge, cpu) / windows-job (gh)
• windows (windows.4xlarge, cpu) / windows-job (gh)

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pytorch/vision / wheel-py3_8-cpu-aarch64 (gh)

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[pmeier]

Thanks Nicolas! Was thinking the same a few days back.