[docs][card] Improve responsiveness in MediaCard/ImgMediaCard demo

[Samii2383]

Updates MediaCard.tsx and ImgMediaCard.tsx to make media responsive:
Use component="img" with sx={{ width: '100%', height: 'auto', aspectRatio: '16 / 9' }} to prevent overflow on small screens.
Add loading="lazy" and proper alt casing.
Scope: docs examples only; no package code changed.

[Samii2383]

Hi! This is a docs-only improvement to two Card demos to behave better on small screens.
Switches CardMedia to fluid sizing with sx={{ width: '100%', height: 'auto', aspectRatio: '16 / 9' }} and adds loading="lazy".
Prevents image overflow in mobile layouts while keeping the demo visually consistent.
Scope: examples only; no package code touched, no changeset needed.
Happy to adjust the aspect ratio or use another recommended pattern (e.g., objectFit: 'cover') if you prefer.

[mui-bot]

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[ZeeshanTamboli]

Use component="img" with sx={{ width: '100%', height: 'auto', aspectRatio: '16 / 9' }} to prevent overflow on small screens.

@Samii2383 Can you give a reproduction perhaps in the form of screenshots or a screen recording where it overflows on small screens?