Add a version field in doctave.yaml

[maloleroy]

Maybe we could create a new version field in doctave.yaml ? This field would be optional, meaning that if it is not set, no version appears (like it is now). If it is set, it could be displayed in the upper left-hand corner, maybe like in one of these examples:

• Directly with the title
  
• Under the title
  
• Next to the title
  

I recommend making this yaml field a string rather than an int or a float, to handle versions like 3.0-alpha easily.