docs: split v3 migration guide

[janechu]

Pull Request

📖 Description

Splits the FAST 3.x migration guide into a primary core migration path plus two focused child paths:

• Core FAST Element migration guidance for runtime requirements, path exports, define()/compose() migration, registry lookups, FAST global changes, debug setup, dynamic stylesheet behavior removal, and boolean attribute semantics.
• Hydration and SSR migration guidance for fast-ssr/SSR marker output, opt-in hydration, removed hydration side-effect imports, hydration promises, and prerender/hydration state.
• Declarative HTML migration guidance for the removed @microsoft/fast-html package, RenderableFASTElement, TemplateElement, TemplateOptions, declarative attribute mapping, event handler syntax, and prepare() migration.

👩‍💻 Reviewer Notes

Please review whether the split matches the migration paths a developer would choose when upgrading component logic, SSR/hydration, or declarative HTML usage.

📑 Test Plan

• git diff --check
• npm run checkchange
• cd sites/website && npx @11ty/eleventy --output=build --input=src
• Generated page link scan for stale .md migration cross-links

✅ 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.

Agents

• I have linked to an existing issue in this project that this change addresses
• I have read the skills
• I have read the DESIGN.md file(s) in packages relevent to my changes
• I have updated the DESIGN.md file(s) in packages relevent to my changes