docs: migrate to nextra@4

[zettca]

migrate to nextra@4:

• move /pages -> /content
• add "use client"; in components that need it
• inject "use client"; in UI Kit packages in Nextra's config (if we want to explicitly support Next App Router/RSC in UI Kit, we should add these in the packages directly)
• add Playground features to Header:
  • almost all instances use <Header><Playground> together
  • getComponentData() only needs to be called once
• change decorator to decoratorClassName to work server-side
• add new Pagefind search:
  • more docs coverage (includes text in .tsx files)
  • control over what is shown (data-pagefind-ignore)
• refactor Phosphor icons to use className="i-ph-*" so they can be built fully server-side
• review global styles (many static classnames were dropped)