docs: rewrite built-in directives section of Template Syntax

[kapunahelewong]

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
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

1. Rewritten documentation on the Built-in Directives section prose.
2. Rebuilt example that is standalone so that it can be its own example when Built-in Directives is pulled out as its own doc.
3. Example is more accessible (added/edited markup to help screen readers).
4. Example is now item themed rather than hero themed.
5. New e2e tests for the example.

Does this PR introduce a breaking change?

• Yes
• No

Other information

This is the 7th PR of rewrites of the Template Syntax page. They all work now in Template Syntax and and are prepped for separating Template Syntax into smaller docs, each with its own example.

[mary-poppins]

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[jenniferfell]

@kapunahelewong Hi. See #27564. Can you work in something here about ngif else? Or make sure there are links to the API ref pages (which Judy has updates for in a separate PR)? Thanks!

[mary-poppins]

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[brandonroberts]

Caretaker note: The Codefresh job is unrelated and still pending. I am using a global approval

[AndrewKushnir]

@kapunahelewong, it looks like this PR now has a conflict with master branch after merging PR 25561 that also updated .github/CODEOWNERS file. Could you please rebase this PR and resolve conflict(s)? Feel free to add "merge" label back once PR is updated. Thank you.

[mary-poppins]

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