Add PortalContentContainer and portal system documentation

Add PortalContentContainer, a PortalContentBase subclass that hosts
arbitrary child controls inside portal overlays with layout, mouse
routing, keyboard delegation, and focus tracking. Register it as a
self-painting container in LayoutNodeFactory.

Add comprehensive PORTAL_SYSTEM.md guide covering architecture,
positioning, content types, routing flows, and usage examples.
Link from index.html, CONTROLS.md, DOM_LAYOUT_SYSTEM.md, and
RENDERING_PIPELINE.md.

Author: nickprotop