Address feedback on platform-provided mixins explainer (3)

[anaskim]

Updates

• Introduce ObservableArray for mixinsList.
• Renames "Mixin" to "Behavior".
• Adds CSS pseudo-class matching clarifications.
• Adds clarity on what the platform provides vs what the developer has to do in terms of exposing properties and attribute reflection.
• Adds a section for open questions offering alternatives to manual vs automatic exposure of attributes.
• Add tracking issue.
• Few nits.