docs: use new nuxi module add command in installation

[danielroe]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

This updates the documentation to use the nuxi module add command which should simplify docs a bit and also improve user experience as there's no need to add to nuxt.config manually.

It's documented here.

I may have missed a few spots in the documentation as I'm doing this across the modules ecosystem assisted by the power of regular expressions ✨, so I'd appreciate a review 🙏

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[netlify]

✅ Deploy Preview for nuxt-strapi-module ready!

Name	Link
🔨 Latest commit	24bdfeb
🔍 Latest deploy log	https://app.netlify.com/sites/nuxt-strapi-module/deploys/6616ae861f3d090008e012c3
😎 Deploy Preview	https://deploy-preview-407--nuxt-strapi-module.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.