Add BFF pattern pivot

[guardrex]

Fixes #34927

cc: @mihirpatelipsos ... I'm adding a pivot to the BWA+Entra article for the BFF pattern.

Sample app: https://github.com/dotnet/blazor-samples/tree/main/9.0/BlazorWebAppEntraBff

UPDATE (3/20): Awaiting final offline discussion to conclude.

NOTE TO SELF 🦖

Immediately after this goes LIVE, update the sample repo README.md file with the sample entry and cross-link to the live article.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/blazor/call-web-api.md	aspnetcore/blazor/call-web-api
aspnetcore/blazor/security/blazor-web-app-with-entra.md	aspnetcore/blazor/security/blazor-web-app-with-entra
aspnetcore/blazor/security/blazor-web-app-with-oidc.md	aspnetcore/blazor/security/blazor-web-app-with-oidc
aspnetcore/blazor/security/index.md	aspnetcore/blazor/security/index
aspnetcore/zone-pivot-groups.yml	aspnetcore/zone-pivot-groups

[mihirpatelipsos]

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