chore: Update Next.js, Nextra versions

This is sort of a to-do since we are on outdated versions of Next(ra) and could really do with updating them.

That being said, this seems like a very large job since there are quite a lot of fundamental breaking changes that don't have a clear migration path...

Check the package.json for current Next.js and Nextra versions. Review the Next.js and Nextra changelogs for breaking changes. Assess the migration path by examining the documentation site's structure and configuration files. Determine what updates are needed in dependencies, configuration, and components.

索引モデルが issue の本文から書いたものです。