docs(fast-element): fix v3 migration guidance

[janechu]

Pull Request

📖 Description

• Fixes FAST Element v3 migration guidance for hydration imports, including deferHydrationAttribute from @microsoft/fast-element/hydration.js.
• Reconciles package and website migration docs for hydration callbacks, removed hydration helpers, and removed API replacements.
• Clarifies that FAST Element v3 does not use or polyfill idle callbacks and that async updates require requestAnimationFrame.

👩‍💻 Reviewer Notes

Docs-only change focused on v3 migration guidance consistency.

📑 Test Plan

• git diff --check
• npx beachball check --scope "!sites/*" --changehint "Run 'npm run change' to generate a change file" --no-fetch --branch origin/releases/fast-element-v3
• npx @11ty/eleventy --output=build --input=src --quiet from sites/website
• npm run biome:check was attempted; Markdown docs are ignored by the current Biome configuration.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.