Write architecture documentation

[TimLariviere]

We need to write all the decisions we made for the architecture of v2, including details on:

• [ ] Add glossary of all the internal names (Widgets, Scalar/Widget/WidgetCollection attributes, Program, ViewAdapter, Reconciler, ViewNode, etc.)
• [ ] Explain UI concepts (UI tree vs Virtual tree)
• [ ] Explain view building process (Widget, attributes, definitions, CE builders)
• [ ] Explain diffing process (Reconciler and ViewNode)
• [ ] Explain MVU lifecycle (Program, Runners and ViewAdapters)
• [ ] Explain feature MapMsg
• [ ] Explain feature Memo (mostly copy-paste #36)
• [x] Explain virtualized collections support (done #9)