feat: add Docusaurus redirect mapping for stable and placeholder URLs

[abose]

🚀 Summary

This PR implements the Phoenix “App Links” redirect system using @docusaurus/plugin-client-redirects (v3.7.0).
It introduces a centralized app-links.js file that defines stable, version-independent URLs used by both the Phoenix app and the documentation site.

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📘 Overview

The new system allows Phoenix desktop and browser apps to reference stable URLs such as:

https://docs.phcode.dev/app-links/*

These URLs can be redirected to any internal or external destination and can also serve as placeholder links before actual documentation pages are written.

This makes it safe to wire app-level links early and restructure documentation later without breaking links.

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🧭 Use Case

• Acts as a stable index of all official URLs referred to by the Phoenix app or docs.
• Enables placeholder URLs before docs are written.
• Future-proofs link sharing and avoids link rot during documentation restructuring.

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⚙️ Behavior

• npm run start: Dev server does not generate redirect pages.
• npm run build: Static redirect pages are generated.
• npm run serve: Serves the built site with working redirects.

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🧱 Schema

Each entry follows:

{ from: string, to: string }

• from → must begin with /app-links/...

• to → can be:

  • an internal docs path
  • an external URL
  • a temporary placeholder for future docs

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💡 Example Entries

{
  from: '/app-links/extn-line-height',
  to: '/docs/editing-text#line-height',
},
{
  from: '/app-links/google-search',
  to: 'https://google.com',
}

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✅ Testing

1. Run npm run build
2. Run npm run serve
3. Visit https://localhost:3000/app-links/<any-entry>
4. Confirm redirect to the correct destination.