Missing docs #88

List of stuff we're missing documentation for:

• update add-to-existing to start 2
• SolidBaseConfig
• Creating themes:
  • ThemeDefinition
  • Reusing default theme components
  • Mdx components
  • [data-theme=]
  • Creating custom dynamic layouts (like landing from default theme)
• Markdown syntax:
  • Rework page layout (don't like how the sections are split up)
  • package- & its config
  • code block file=
  • code block frame=
  • code block tab
  • issue autolink
  • tab groups :::tab-group
• Default theme:
  • sidebar & config
  • all frontmatter settings
  • Hero & Features (used in landing page)
  • toc & its config
• I18n:
  • config
  • file structure

Feel free to edit this list as we go and add stuff