docs(forms): remove remaining experimental framing from Signal Forms docs - #71116

PR Checklist

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• 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?

Issue Number: N/A

Signal Forms APIs were graduated to public API in v22 (#68581), and the experimental framing was subsequently removed from the package documentation in #68769 and #69305.

There are still two places in adev that describe Signal Forms as not stable:

• The last step of the Signal Forms tutorial says that Signal Forms is experimental and tells readers to check the documentation for API updates.
• The Signal Forms overview recommends Reactive Forms for applications that need production stability guarantees.

What is the new behavior?

The tutorial still points readers to the official documentation, but no longer describes Signal Forms as experimental.

The overview still recommends Reactive Forms for existing Reactive Forms applications, but no longer suggests that Signal Forms lacks production stability guarantees.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Docs-only change in adev/src/content/tutorials/signal-forms/steps/6-next-steps/README.md and adev/src/content/guide/forms/signals/overview.md.