docs: supplement responsive usage

[a1mersnow]

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Try to help beginners use <NuxtImg>'s responsive features.

I changed the example below <NuxtImg>'s sizes property:

<!-- before -->
<NuxtImg
  src="/logos/nuxt.png"
  sizes="100vw sm:50vw md:400px"
/>
<!-- after -->
<NuxtImg
  src="/logos/nuxt.png"
  sizes="300px sm:50vw md:400px"
/>

And I recommend giving a pixel value for the default size.

If you use 100vw as the default size, you will get srcsets of 1w and 2w which do not make sense.