[WXP] (runtimes) Clarify the article for setting up shared runtimes

[Rick-Kirkham]

This update moves the overview passage from the end of the article to the beginning.
It smooths out the dual purpose of the article: (1) Providing a step-by-step for a continuing example, and (2) providing generic advice for reconfiguring an existing add-in for the shared runtime.
It improves the support of unified manifest and the distinction between the two manifest types.

Fixes 5751

[learn-build-service-prod]

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[learn-build-service-prod]

Learn Build status updates of commit 966331b:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/develop/configure-your-add-in-to-use-a-shared-runtime.md	✅Succeeded	View
docs/images/custom-functions-in-browser-runtime.png	✅Succeeded	n/a (file deleted or renamed)

For more details, please refer to the build report.