Update Angular integration guide to modern patterns (v20+)

[sequba]

Context

The Angular integration guide was using outdated patterns (NgModule, RxJS BehaviorSubject, async pipe, OnDestroy lifecycle hook). Angular 20+ has introduced new idiomatic approaches (standalone components, signals, @if/@for control flow, inject(), DestroyRef) that are now the recommended way to build Angular applications.

This update modernizes the documentation to reflect current Angular best practices while maintaining clarity for developers using the library.

Changes

• Signals instead of RxJS: Replaced BehaviorSubject and Observable with Angular signals for simpler state management and automatic change detection
• Modern component syntax: Updated examples to use standalone components with @Component decorator and inject() for dependency injection
• Control flow syntax: Replaced *ngIf and *ngFor with @if and @for built-in control flow
• Cleanup pattern: Replaced implements OnDestroy / ngOnDestroy() with inject(DestroyRef).onDestroy() callback pattern
• OnPush change detection: Added ChangeDetectionStrategy.OnPush to the component example for better performance
• Clarified scope: Updated provider scope explanation to reflect modern terminology
• Added context note: Included a tip at the end noting that the linked Stackblitz demo uses older patterns and directing readers to prefer the updated snippets for new projects
• RxJS bridge note: Added guidance for projects that still use RxJS streams, explaining how to use toSignal() at component boundaries

How did you test your changes?

Documentation review only. The code snippets provided are syntactically correct Angular 20+ code following official Angular patterns and best practices.

Types of changes

• Change to the documentation

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• My changes require a documentation update.

https://claude.ai/code/session_01KG37ikpGHrkUUgcGKUotnW

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.18%. Comparing base (d6a316c) to head (33703c4).

Additional details and impacted files

@@                             Coverage Diff                              @@
##           feature/hf-122-framework-integration-guides    #1658   +/-   ##
============================================================================
  Coverage                                        97.18%   97.18%           
============================================================================
  Files                                              174      174           
  Lines                                            15086    15086           
  Branches                                          3223     3223           
============================================================================
  Hits                                             14662    14662           
  Misses                                             424      424           

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[github-actions]

Performance comparison of head (33703c4) vs base (d6a316c)

                                     testName |   base |   head | change
------------------------------------------------------------------------
                                      Sheet A | 499.12 | 496.76 | -0.47%
                                      Sheet B | 159.47 | 160.53 | +0.66%
                                      Sheet T |  142.7 | 141.31 | -0.97%
                                Column ranges | 472.42 | 471.66 | -0.16%
Sheet A:  change value, add/remove row/column |  15.78 |  15.85 | +0.44%
 Sheet B: change value, add/remove row/column | 135.16 | 130.14 | -3.71%
                   Column ranges - add column | 149.86 | 147.32 | -1.69%
                Column ranges - without batch | 469.94 | 444.72 | -5.37%
                        Column ranges - batch | 119.67 |  116.1 | -2.98%