fix(docs): add warning about using `ssr: false` with Content

[mdelally]

Add warning about using Nuxt Content with SPA static generation (ssr: false in Nuxt Config) on the Installation guide in the documentation.

🔗 Linked issue

Issue #1229

❓ Type of change

Documentation edit

• [x] 📖 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

The following change to the Installation page of the docs makes a note that Nuxt Content currently requires ssr: true to be set due to static SPA (ssr: false) being incompatible.

📝 Checklist

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