Add documentation

[neuropilot-captain]

1. Add README page to MediaTek backend
2. Improve documentation
3. Add documentation for ExecuTorch online doc

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6883

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❗ 1 Active SEVs

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• [DomainsOnly] Jobs fail with GLIBC version not found

❌ 3 New Failures

As of commit cf8a519 with merge base 39e5b91 ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  ##[error]The process '/usr/bin/sh' failed with exit code 1
• pull / test-llama-runner-qnn-linux (fp32, qnn) / linux-job (gh)
  Invalid image name executorch-ubuntu-22.04-qnn-sdk
• pull / test-llava-runner-linux / linux-job (gh)
  test_prefill_logits

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[cccclai]

Wow there were so many typos....

[cccclai]

Does it include all Linux or some of them? Will be good to list out the OS that was tested..

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