Add help text that highlights major changes for site builders, like layout system and text editing settings

[irinaz]

BackdropCMS has several major changes that often are stumbling block for site builders after migration is complete. Among them primary are

• Layouts system
• Text editing settings (combines filters configuration and formats configuration from D7)

It would be helpful to add help text with links to documentation - need to better understand where in UI flow this can be done best.