Docs: Add vNext release notes and update README with vNext highlights and assets

[bkaankose]

Motivation

• Surface the upcoming vNext work and highlight major product areas (Mail, Calendar, Contacts, performance and WinUI polish) directly in the repository README.
• Provide a human-readable changelog that summarizes the scope and important commits merged into feature/vNext relative to main.
• Improve README accessibility and visuals by adding alt text and new illustrative assets to better communicate feature pillars and highlights.

Description

• Expanded README.md to describe Wino as a "mail and calendar" client, added a vNext Release Highlights section, enriched the features list with emoji-driven bullets, and added image alt attributes for accessibility.
• Added RELEASE_NOTES_vNext.md which documents major merged areas, detailed improvements, and a commit-by-commit one-line summary of changes included in the vNext work.
• Added two new SVG assets (docs/readme-assets/vnext-highlights.svg and docs/readme-assets/feature-pillars.svg) used by the updated README for visual summaries.
• Minor README cleanup of links and download badge markup for a cleaner and more consistent presentation.

Testing

• No automated tests were executed as part of this PR because the change is documentation and asset additions only.
• Existing CI/test workflows remain unchanged and will continue to run for code changes in other PRs.

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Codex Task