docs: define next UI requirements

[ChuxiJ]

Summary

• Add docs/en/NEXT_UI_REQUIREMENTS.md with product goals, non-goals, user tiers, first-run requirements, navigation, progressive disclosure, help/recovery coverage, and implementation constraints for a future beginner-friendly UI.
• Link the requirements doc from the README, English docs sidebar, and UI support baseline.

Issue

Closes #1180.

Validation

• git diff --check
• test -f docs/en/NEXT_UI_REQUIREMENTS.md && test -f docs/en/UI_SUPPORT.md
• uv run python -m unittest acestep.ui.gradio.help_content_md_test acestep.ui.gradio.help_content_misc_test

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/next-ui-requirements

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