"Overview of Microsoft Edge extensions": aka add-ons, link to user docs

[mikehoffms]

Rendered article for review:

• Overview of Microsoft Edge extensions
  • https://review.learn.microsoft.com/microsoft-edge/extensions-chromium/index?branch=pr-en-us-3245
  • Before/Live

This PR makes the following changes:

• Defines 'add-on' as synonym of 'extension'.
• Introduces add-ons website at start of article.
• Links to user docs that introduce extensions & using them.
• In section about signing up to publish at Edge Add-ons site:
  • Introduces add-ons website & links to it.
  • Clarifies that "the store" means the Edge Add-ons website.
  • Makes link text match destination title.

AB#53096784

[learn-build-service-prod]

Learn Build status updates of commit 92b9716:

✅ Validation status: passed

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[learn-build-service-prod]

Learn Build status updates of commit 0886752:

✅ Validation status: passed

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• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 20e1b5e:

✅ Validation status: passed

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[captainbrosset]

Approving because this is very useful to readers.

[learn-build-service-prod]

Learn Build status updates of commit eac08d5:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/extensions-chromium/index.md	✅Succeeded	View

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For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 6175351:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/extensions-chromium/index.md	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel