Extensions landing page

[mikehoffms]

Rendered landing page for review:

• https://review.learn.microsoft.com/microsoft-edge/extensions-chromium/landing/?branch=pr-en-us-3265
• Planned live
• TOC before/live

To force-load the revised TOC:
F12 > right-click "Refresh" > "Empty cache and hard refresh" > F12
Open different branches in different windows.

This PR makes the following changes:

• Added a landing page for Extensions, as a destination that's not a particular article; represents whole set of articles as a URL to give to users.
• Modified the TOC, so that bucket names use concrete keywords that match article title keywords:
  • Broke out the "Publish an extension" toc bucket into "Register as an extension developer" and "Publish an extension".
    • Helps group Publish articles, and supports structuring the relation (future toc design) of those articles.
  • Broke out the "Manage and maintain an extension" toc bucket into "Update an extension" and "Analytics and user reviews"
    • To surface the key words of the contained articles' titles, instead of abstract synonyms.
    • Matches scope of landing page cards of uniform length / article-count.
• Changed title from:
  Automatically update an extension in Microsoft Edge
  to:
  Set an extension to automatically update
  • Partly improves lead-in summary flow in that article.

AB#53967047

[learn-build-service-prod]

Learn Build status updates of commit 21d9b11:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/extensions-chromium/index.md	✅Succeeded	View
microsoft-edge/extensions-chromium/landing/index.yml	✅Succeeded	View
microsoft-edge/extensions-chromium/publish/api/addons-api-reference.md	✅Succeeded	View
microsoft-edge/extensions-chromium/publish/api/using-addons-api.md	✅Succeeded	View
microsoft-edge/extensions-chromium/publish/auto-update.md	✅Succeeded	View
microsoft-edge/toc.yml	✅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

[captainbrosset]

I don't have comments about the landing page itself, which I think is great.
It's very useful to have a landing page that we can link users to, as an entry point to the extensions docs.
I did leave a couple of comments about other changes.

[learn-build-service-prod]

Learn Build status updates of commit 5b4ed24:

✅ Validation status: passed

File	Status	Preview URL	Details
microsoft-edge/extensions-chromium/index.md	✅Succeeded	View
microsoft-edge/extensions-chromium/landing/index.yml	✅Succeeded	View
microsoft-edge/extensions-chromium/publish/api/addons-api-reference.md	✅Succeeded	View
microsoft-edge/extensions-chromium/publish/api/using-addons-api.md	✅Succeeded	View
microsoft-edge/extensions-chromium/publish/auto-update.md	✅Succeeded	View
microsoft-edge/toc.yml	✅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