chore: remove duplicate mdast-util-to-string in site package

[icyJoseph]

Description

Remove duplicate mdast-util-to-string, it was present in devDependencies and dependencies.

• I have read the Contributing Guidelines and made commit messages that follow the guideline.
• I have run pnpm format to ensure the code follows the style guide.
• I have run pnpm test to check if all tests are passing.
• I have run pnpm build to check if the website builds without errors.
• I've covered new added functionality with unit tests if necessary.

[vercel]

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Project	Deployment	Preview	Updated (UTC)
nodejs-org	Ready	Preview	Oct 29, 2025 9:28am

[bmuenzenmeyer]

thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.61%. Comparing base (aea78cb) to head (1a9bb3f).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8287      +/-   ##
==========================================
+ Coverage   76.57%   76.61%   +0.04%     
==========================================
  Files         117      117              
  Lines        9733     9746      +13     
  Branches      328      328              
==========================================
+ Hits         7453     7467      +14     
+ Misses       2278     2277       -1     
  Partials        2        2              

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