docs: add animated hero image to README

[livlign]

Summary

Proposes an animated hero at the top of README.md that shows MarkItDown converting representative file types (PDF, PowerPoint, Word, Excel, HTML, images, audio) into LLM-ready Markdown in a single loop.

• Adds docs/images/hero.gif (1200×675, ~2.3 MB, 20s loop)
• Adds docs/images/hero.html — the editable source (single self-contained HTML file). Future maintainers can open it in any browser, tweak, and re-render.
• Adds docs/images/README.md — documents the Puppeteer + ffmpeg pipeline so the GIF can be regenerated without archaeology.
• Inserts a single ![…] line in README.md between the badges row and the > [!IMPORTANT] security callout — placement keeps the security notice above the fold on mobile.
• No other changes.

Test plan

• Open README.md on GitHub and confirm the hero renders between the badges row and the IMPORTANT callout, and that the callout remains prominent.
• Easy revert: git revert the single commit.

Feel free to close if it isn't aligned with Microsoft's brand direction for the repo — happy to iterate or drop it.

[livlign]

Just flagging for transparency: this is a personal contribution — not submitted on behalf of a company or affiliated team. Happy to iterate on feedback or close it out if the direction doesn't fit.

[livlign]

@microsoft-github-policy-service agree