Update the best practice documentation for QML

We should at the very least discourage unqualified lookups, and provide alternatives there:

• Using functions with parameters instead of injected signal parameters
• Use required properties to avoid injecting model roles
• Qualifying property accesses instead of relying on lookups in the root scope
  Notably, we shouldn't encourage the use of setContextProperty in the "good" part of the page.

We should also further investigate what else can be mentioned there.