docs(api): add useHydration composable

[Krutie]

🔗 Linked issue nuxt/nuxt#13653

❓ 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 like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Added API documentation for useHydration composable

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

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[tylerforesthauser]

Firstly, many thanks for everything you've done and continue to do for the community, Krutie! Secondly, is it possible to add an example to this as well? This functionality has inexplicably confused me since I first came across it. 😵‍💫

[pi0]

Thanks for the PR @Krutie <3

I'm merging it as is but there is a chance we remove useHydration composable before 3.0.0 release. It was initially made to help making store implementation but modules can always write same logic.