Update the AI codegen resources for Angular v22:

• best-practices.md: note that OnPush is the default in v22+ (don't set it explicitly), recommend Signal Forms, and recommend the @Service decorator.
• llms.txt: add a Signal Forms reference, the httpResource guide, and an Accessibility section linking the Angular Aria overview.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

The "Build with AI" resources (best-practices.md and llms.txt) predate Angular v22. They don't mention that OnPush is now the default change detection strategy, don't reference Signal Forms or the @Service decorator, and llms.txt has no links to Signal Forms, httpResource, or Angular Aria.

Issue Number: N/A

What is the new behavior?

The resources are updated for Angular v22:

• best-practices.md notes OnPush is the v22+ default (don't set it explicitly), recommends Signal Forms for new forms, and recommends the @Service decorator over @Injectable({providedIn: 'root'}).
• llms.txt adds a Signal Forms reference under Forms, the httpResource guide under Loading Data, and a new Accessibility section linking the Angular Aria overview.

Does this PR introduce a breaking change?

• Yes
• No

Other information

All added links were verified against adev/src/app/routing/navigation-entries/index.ts.

I focused on the most obvious v22 additions (Signal Forms, the OnPush default, the @Service decorator, httpResource, and Angular Aria) and intentionally kept both files minimal — best-practices.md as a short rules file and llms.txt as references only. If the team thinks other v22 features belong here (e.g. additional template enhancements, router updates, or AI tooling references), I'm happy to add them.