Cleanup of Workbox build examples

[jeffposnick]

R: @malchata
CC: @tropicadri

This more of less matches the inline examples we added to the TSDocs in GoogleChrome/workbox#3038, and links to the relevant reference docs for the full configs in a way that's hopefully easier to find.

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[chrome-devrel-review-bot]

Hello! This is an automated review by our custom reviewbot. It updates automatically when code or GitHub comments in this pull request are created or updated.

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site/en/docs/workbox/modules/workbox-build/index.md

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site/en/docs/workbox/modules/workbox-cli/index.md

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site/en/docs/workbox/modules/workbox-webpack-plugin/index.md

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[chrome-devrel-review-bot]

Hello! This is an automated review by our custom reviewbot. It updates automatically when code or GitHub comments in this pull request are created or updated.

Requested changes

If there are any common problems with the content files you created or modified, they will be listed here.

site/en/docs/workbox/modules/workbox-build/index.md

• This file passed all of our automated Markdown audits.

site/en/docs/workbox/modules/workbox-cli/index.md

• This file passed all of our automated Markdown audits.

site/en/docs/workbox/modules/workbox-webpack-plugin/index.md

• This file passed all of our automated Markdown audits.

[malchata]

Minor typos, otherwise, LGTM!