docs(web): add iframe vs DOM embedding comparison and container CSS styling to embedding-flutter-web

### Summary The page [`docs.flutter.dev/platform-integration/web/embedding-flutter-web`](https://docs.flutter.dev/platform-integration/web/embedding-flutter-web) explains full-page mode, `iframe` embedding, and multi-view embedded mode (`multiViewEnabled`), but lacks comparison guidance and DOM container setup rules. ### Proposed Additions 1. **`iframe` vs Direct DOM Embedding Trade-offs**: Add a comparison matrix guiding developers on when to choose `iframe` (security sandbox, isolated JS/CSS context, isolated memory) vs Direct DOM `hostElement` embedding (shared memory, direct JS interop, seamless styling). 2. **Container CSS Styling Requirements**: Document mandatory CSS styling properties (`width`, `height`, `position: relative`, `overflow: hidden`) required on host DOM elements. Unstyled host elements default to `height: auto` (0px), causing `CustomElementDimensionsProvider` to compute view height as 0 and collapse the Flutter view.