Add client-side redirects

[scott-klaytn]

✨ Proposed Changes

• Add client-side redirects using a plugin
  When we change the structure of our Docs site, some links break. We check and fix internal links, but links shared outside the docs aren’t always updated. This can lead to 404 errors and a poor user experience. Recently, a user complained about a broken link from the old Learn section. To fix this, it's needed to set up redirect rules for moved pages using a plugin.
• Change the color of SEND API REQUEST button from white to black for better contrast
  

🗂️ Types of changes

• Minor Issues and Typos
• Major Content Contribution
• Others (e.g. platform-specific changes)

✅ Checklist

• I have read the CONTRIBUTING GUIDELINES.
• If following Standard Workflow: I have linked the approved issue below.
• If submitting without a prior issue (Maintainer/Regular): I have provided detailed context in the "Proposed Changes" section above.
• My changes render correctly (if applicable, run npm run build locally).
• I have added necessary documentation updates (if appropriate).
• Any dependent changes have been merged and published.

📌 Related issues

💡 Further comments

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