Add https://analytics.dwyl.com to Nextra Demo

[nelsonic]

Todo

• [x] Deploy the Nextra Demo preferably as GitHub Pages :octocat:
  • [x] but if the auth requires a Vercel deployment, fork the repo and do it on your own account (document the steps) 🙏
• [x] create an analytics.md file in the root of this repo. 🆕
• [x] Give a basic intro and explain how to use Plausible Analytics in a Nextra/Next.js site 📝
• [x] Link directly to where you have documented this in: https://github.com/dwyl/learn-analytics/tree/main/plausible#3-optional-monitoring-a-nextjs-website 🔗
• [x] Confirm that it's working by sharing a screenshot of the Analytics dashboard.