docs: Add supported content types to sample code

[otusoa]

🔗 Linked issue

harlan-zw/nuxt-seo#492

❓ 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

import { defineCollection, defineContentConfig } from '@nuxt/content'
import { asSitemapCollection } from '@nuxtjs/sitemap/content'

export default defineContentConfig({
  collections: {
    content: defineCollection(
      asSitemapCollection({
        type: 'page',
        source: '**/*.{md,yml,yaml,json,csv}', // now supports all supported file formats
      }),
    ),
  },
})

Key Changes

I modified the source property to include all supported file formats: source: '**/*.{md,yml,yaml,json,csv}'.

[otusoa]

Hello!
This is the person who submitted the issue earlier.

I've created the pull request now—please merge it when you have time.