Chore: upgrade Next.js, clean, and add some features

[Mcastres]

This PR:

• Upgrade Next.js to 12
• Remove unnecessary packages
• Replace default article richtext editor with ckeditor 5
• Replace GitHub button with the react one
• Implement SEO on the frontend which was broken
• Dynamically import DZ components on the front-side to increase performance.

[ivanThePleasant]

Ok, I didn't see any issues or anything, and thanks for changing the Node version. LGTM, unless you want to wait for Fabien's review as well 👍

[Mcastres]

No, it will be enough! Thanks @ivanThePleasant 🙏