Update documents for buffer allocator

[neuropilot-captain]

Summary

1. Update MediaTek backend documents for the decoupled buffer allocator.
2. Follow backend template.
3. Remove unnecessary instructions.

Fixes #8532

@pytorchbot label "partner: mediatek"

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11467

• 📄 Preview Python docs built from this PR

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✅ No Failures

As of commit 4d7f35f with merge base e02ca41 ():
💚 Looks good so far! There are no failures yet. 💚

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[github-actions]

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[cccclai]

It's helpful to know the supported SoCs. Can we keep/update it?