Update Why Flutter whitepaper PDF, Open Graph image, and section header alignment

[abdallahshaban557]

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

• Replaces sites/www/content/why-flutter/flutter-whitepaper-2026.pdf with the updated Flutter whitepaper PDF.
• Adds sites/www/content/why-flutter/images/why-flutter-sharing.png and sets socialImage in sites/www/content/why-flutter/index.md for the Open Graph and Twitter card metadata.
• Centers .stacked-header and .why-flutter-sub in _why_flutter.scss so the headers for the Design fidelity, Success cases, and Resources sections are centered on the /why-flutter page.

Issues fixed by this PR (if any):

None.

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

None.

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.

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 944843a):

https://flutter-dev-230821--www-pr13969-update-why-flutter-ass-1b07jsrd.web.app

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit cb07cd0):

https://flutter-docs-prod--docs-pr13969-update-why-flutter-ass-vbh13dph.web.app

[gemini-code-assist]

Code Review

This pull request updates the 'Why Flutter' page by adding a social image to the front matter and applying layout adjustments to the .stacked-header and .why-flutter-sub sections, specifically centering the header and horizontally centering the sub-section. I have no feedback to provide.

[gemini-code-assist]

Code Review

This pull request adds a social sharing image to the 'Why Flutter' page front matter and updates the CSS styling to center the header and sub-elements. The review feedback suggests using a root-relative path for the social image to avoid potential 404 resolution issues for crawlers.

[MuthuGCodes]

LGTM