Copy symbolic links in the assets as actual files

[aYukiSekiguchi]

Summary

This PR addresses issue #1966 by modifying the asset copying function to handle symbolic links correctly.

Details

The commit f4a46cb changed the asset copying function from File.copy/3 to File.cp_r!/3, which altered the handling of symbolic links.

To resolve this, we add the option dereference_symlinks: true to File.cp_r!/3 as advised by José Valim.

Additional Changes

• Ran mix fix to address linting issues present on the current main branch. If necessary, I can create a separate PR or revert this commit based on reviewer feedback.

Testing

• Passed mix lint
• Passed mix test
• Run mix build and check some pages in doc/index.html
• Confirmed that all assets are actual files in the reproduced repository antikythera

Please review and let me know if any further changes are needed.

[josevalim]

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