docs: add FormField/FormRoot imports + move comment with ``'s

[michael-small]

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?

1. Backticks in inline template comments were breaking the file.
2. The section dedicated to FormRoot was missing it's TS import
3. All TS imports are in the validation styling example, but the template import is missing

Issue Number: N/A

What is the new behavior?

1. Rather than removing the backticks, the comment was placed between the component's imports and the template, still effectively above the reference to the formField. I think it looks fine and makes sense.
   • I also broke the comment into two lines because it was rather long IMO
2. The section dedicated to FormRoot has its TS import too
3. One extra line to import FormField to the HTML makes the example file work as a valid component file in its entirety. I know the docs sometimes cut certain TS/HTML imports if they were covered and not the main focus, but IMO one line to make the full block compile is nice.

Does this PR introduce a breaking change?

• Yes
• No

Other information

I imagine point 1 and point 3 may be a bit contentious.

Also congrats on the announcement about going stable!