docs: fix Vercel deployment failures by async loading Sandpack component

[kangju2000]

This PR fixes Vercel deployment failures by implementing async loading for the Sandpack component in our vitepress theme config.

I believe the issue is caused by vitepress-plugin-sandpack's Sandbox component using DOM APIs on the server side. This change ensures Sandpack only loads client-side, preventing SSR-related errors.

We can't verify the fix in this PR as Vercel deployment doesn't run on PRs. We'll need to merge to confirm the solution works.

[vercel[bot]]

@kangju2000 is attempting to deploy a commit to the Toss Team on Vercel.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.14%. Comparing base (d7247b8) to head (8a5a522).

Additional details and impacted files

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##             main     #358   +/-   ##
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  Coverage   99.14%   99.14%           
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  Files         158      158           
  Lines        1163     1163           
  Branches      301      301           
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  Hits         1153     1153           
  Misses          9        9           
  Partials        1        1           

[raon0211]

Seems that it's not working, but I will open building docs in the pull request. Could you give it another try?