Added readme for Wasm extension

[Conarnar]

Summary

Added readme containing build instructions and API documentation for Wasm extension.

Test plan

N/A

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13131

• 📄 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

There are 1 currently active SEVs. If your PR is affected, please view them below:

• ghstack-mergeability-check and Check labels failing with 'Resource not accessible by integration'

❌ 2 New Failures, 1 Unrelated Failure

As of commit d14d186 with merge base e55bf54 ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• pull / test-moshi-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e7c950310c99754dd968e98f13f74367ca9a91e3115d1f966e4bf9346aa05a4d /exec failed with exit code 1

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

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

• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  AttributeError: '_OpNamespace' 'quantized_decomposed' object has no attribute 'convert_element_type'

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[lucylq]

Thanks for the detailed readme!