docs(service-worker): add staleWhileRevalidate strategy

[ajitsinghkaler]

There is great workaround for implementing staleWhileRevalidate strategy in service-worker by setting strategy to freshness and timeout to 0u. Documented this in service worker config where all other strategies are documented

Fixes #20402

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?

NO staleWhileRevalidate strategy in service worker

Issue Number: #20402

What is the new behavior?

Added staleWhileRevalidate strategy in service worker

Does this PR introduce a breaking change?

• Yes
• No

Other information

[mary-poppins]

You can preview b4b6f67 at https://pr37301-b4b6f67.ngbuilds.io/.

[gkalpak]

Thx, @ajitsinghkaler ✨
I made some suggestions wrt wording (and some extra details). Otherwise, I think this is a nice addition 👌

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[mary-poppins]

You can preview c8fef55 at https://pr37301-c8fef55.ngbuilds.io/.

[ajitsinghkaler]

@gkalpak made the changes. Please have a look.

[gkalpak]

LGTM ✨
Thx, @ajitsinghkaler 👍

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