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. Done! 👍

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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]

[like] Mihir Patel reacted to your message:

[guardrex]

Looks good @mihirpatelipsos! 🎉 We're not going to have a product unit review on this one after discussing it with Halter offline. We'll take community feedback on the article and sample. As soon as this builds, I'll merge it ... then merge it into the live doc set. Thanks again!