[Docs] fix dead links in multiple documentation pages

[mattheliu]

Description

This PR updates the documentation link for torchax to point to the new official site.
Previously, the README referenced an outdated link:
https://jax-torch-interop.readthedocs.io/en/latest
which has now been replaced with the correct and maintained one:
https://google.github.io/torchax/

Why this change:
The original documentation URL was deprecated and no longer reflects the latest updates to torchax. Updating it ensures users are directed to the correct resource.

Implementation details:

• Updated the link in the README and related documentation sections.
• No code logic or functionality was modified.

Future improvements:

• Consider adding link validation in CI to automatically detect broken or outdated URLs.

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Tests

This change only modifies documentation.

• Verified that the new link is valid and accessible.
• Confirmed that the rendered Markdown displays correctly in both GitHub and MkDocs preview.

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Checklist

• I have performed a self-review of my changes.
• I have verified that the new documentation link is correct.
• I have made corresponding updates to the documentation where needed.