docs: update Angular integration examples for modern APIs

[ArturQuirino]

Summary

Updates the Angular integration docs so examples follow current Angular patterns: signal-based viewChild / input() and teardown with inject(DestroyRef) instead of @ViewChild, @Input(), and ngOnDestroy.

What changed

• Basic setup — viewChild.required, AfterViewInit, and inject(DestroyRef).onDestroy() for superdoc.destroy(); SuperDoc is created in ngAfterViewInit so the template ref exists.
• Full component — input.required() / input() for inputs; same viewChild + DestroyRef pattern; document and user read via this.document() / this.user().
• Handle file uploads — Restores full imports at the top of the snippet; same viewChild + DestroyRef cleanup; selector uses this.editorRef().nativeElement.

Why

Keeps the Getting Started Angular page aligned with modern Angular and copy-pasteable for recent projects.

Test plan

• pnpm run check:imports
• pnpm run dev:docs — verify Angular Integration page renders and snippets look correct