feat(demo): improve styling and add italian

[casungo]

This pull request includes several updates and improvements to the i18n demo:

• Revamped the header and language switch components
• Created new reusable components
• Updated page layouts and content
• Added Italian language support
• Removed old test pages
• Updated the README with the latest information

The French translations were machine-generated as a starting point, but should be reviewed by a native speaker before merging (you :P ). The italian translation is fine since I'm native in Italian. Description, author and body fields for blog posts have not been implemented, maybe you can give it a try when you have time. I don't know enough of Astro to do that 😄

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[casungo]

this should cover all of the problems :)

[florian-lefebvre]

Just saw that the language switcher is broken for some reason sigh (nothing to do with the demo don't worry)