doc: Revise and organize Ubuntu-specific build instructions
doc: Revise and organize Ubuntu-specific build instructions

Organize the instructions into sections to separate steps that may
require user decisions.

billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake