Migrate/nextra3

[moklick]

This PR migrates all sites to nextra 3. For this I followed the migration guide: https://the-guild.dev/blog/nextra-3#migration-guide-to-nextra-3

Biggest changes are:

• use _meta.ts/tsx instead of _meta.json and use SidebarTitle in _meta.tsx instead of nextra theme config
• replace deprecated useNextSeoProps with custom head component in nextra theme config

Todos:

• [ ] find out why what's new pages do not have a TOC