Document browser support for service workers

[jbogarthyde]

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?

Behavior of SW apps in browsers that don't support it is not documented.

Issue Number: #21304

What is the new behavior?

Add doc to Service Worker intro.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Content transferred from unfinished PR #22546

[mary-poppins]

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[gkalpak]

A few minor comments. LGTM otherwise 👍

[mary-poppins]

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[kara]

@jbogarthyde Looks like George's comments still need to be addressed, so marking this as cleanup

[jbogarthyde]

oops. incorporated remaining comments.

[mary-poppins]

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[gkalpak]

@googlebot I consent.

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[mary-poppins]

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