Add new design doc for multi-view on iOS

[dkwingsmt]

Description of what this PR is changing or adding, and why:

Add new design doc: https://docs.google.com/document/d/1tcul2KJcZkhXN5ajIcP90p_lSFv-7mAAr0qwBNtiqFQ/edit?tab=t.0

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request adds a new 301 redirect entry for /go/multi-view-on-ios in sites/docs/firebase.json. The feedback suggests reordering the entry to maintain the alphabetical sorting of the redirect rules.

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit c68d8e4):

https://flutter-dev-230821--www-pr13878-dkwingsmt-patch-1-5kr8m9zh.web.app

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit c68d8e4):

https://flutter-docs-prod--docs-pr13878-dkwingsmt-patch-1-95e4g0r9.web.app

[parlough]

Thanks!