docs: update DI in action to recommend new way of defining providers

[sreevani-ship-it]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• 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
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Does not describe the new way of defining providers
Issue Number: N/A

What is the new behavior?

added doc that describes the new way of defining providers

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[mary-poppins]

You can preview 5a0b5e3 at https://pr23076-5a0b5e3.ngbuilds.io/.

[mary-poppins]

You can preview 4fb820c at https://pr23076-4fb820c.ngbuilds.io/.

[alxhub]

That decorator indicates that the Angular DI system will be used to create one or more instances of UserContextService.

[alxhub]

Delete this alert-is-helpful, it's factually incorrect. The l-sub-section following it (about why AppComponent doesn't need Injectable) can also go away.

[mary-poppins]

You can preview 88b8f5a at https://pr23076-88b8f5a.ngbuilds.io/.

[mary-poppins]

You can preview 7385edd at https://pr23076-7385edd.ngbuilds.io/.

[mary-poppins]

You can preview 142084a at https://pr23076-142084a.ngbuilds.io/.

[jenniferfell]

@IgorMinar Would you please approve this? It's asking for someone from io-guide-and-tutorial. Alex R and I have approved. Just need to pass pull-approve test now. Thank you!

[jenniferfell]

@IgorMinar Can you also provide an approval. We need an approval from you or someone else in the guide-and-tutorial group. This PR doesn't touch tutorial files, just ngmodule and DI files, but pull-approve is asking for the guide-and-tutorial group. AlexR has approved. Thanks!

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