Address feedback on platform-provided mixins explainer (3) (#1245)

# 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.

Author: anaskim