Revise search add-ins documentation for SPFx recommendation

[Ashlesha-MSFT]

Category

• Content fix

Related issues

• fixes SPFx Docs Update for content: docs/general-development/search-add-ins-in-sharepoint.md #10828

What's in this Pull Request?

• Added deprecation notice for SharePoint Add-ins in SharePoint Online
• Added recommendation to use SharePoint Framework (SPFx) and Microsoft Graph APIs for new development
• Updated legacy MSDN links to Microsoft Learn paths
• Added notes about deprecated Silverlight-based APIs
• Performed minor documentation cleanup and formatting improvements

[learn-build-service-prod]

Learn Build status updates of commit 88f0fe7:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/general-development/search-add-ins-in-sharepoint.md	✅Succeeded	View

For more details, please refer to the build report.

[learn-build-service-prod]

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