Docs: avoid full-page rerenders on route transitions

## Status
Already handled by the current view layer on `main` — this is now a **documentation task**.

`createRoutesView` renders only the currently opened view via `useOpenedViews`, and nested content is composed through `Outlet` rather than remounting the whole page (`packages/react/lib/create-routes-view.tsx`).

## What's left
- Document how to structure views so transitions don't remount/rerender the whole page.
- Note constraints and recommended patterns (layouts, `Outlet`, stable view identity).
- Write test from the issue atomic-router/atomic-router#93

## Traceability
Original report: atomic-router/atomic-router#93 — https://github.com/atomic-router/atomic-router/issues/93
Related: #57