General layout of the template

[Rockerby]

The welcome dashboard needs completed to act as the template for use with other areas.

• Correct button styling to have a book, and link to the correct place (https://docs.umbraco.com/umbraco-backoffice/)
• Update the copy to make a point about the new back office
• Add supporting comments to the ts file to explain more about the structure and how to extend
• Make a note that the documentation links may change (especially around the UUI backoffice story book references)