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 Implementation of View.memo (dependsOn in Fab v1) #36)
• Explain virtualized collections support (done [Architecture] Support virtualized collections #9)