Aspnetcore.docs: Install nuget

[JohnTraDolta]

Maybe tell the user to install the nuget package
Microsoft.AspNetCore.Authentication.Facebook

@Rick-Anderson edit:

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Document Details

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• ID: f2c6faa2-8c65-073b-4988-672c1aa9690c
• Version Independent ID: d22d616f-907e-ae62-1f43-88ad176a87d4
• Content: Facebook external login setup in ASP.NET Core
• Content Source: aspnetcore/security/authentication/social/facebook-logins.md
• Product: aspnet-core
• Technology: aspnetcore-security
• GitHub Login: @Rick-Anderson
• Microsoft Alias: riande

[guardrex]

All of the social auth topics don't call out their packages. @Rick-Anderson, I'll have some time after the next round of Blazor PRs. If ur busy, I can take this one ... assign me if u want.

[Rick-Anderson]

@guardrex Thanks for the offer. @wadepickett will be updating all of these.

[wadepickett]

I updated Google, the OAuth overview and working on Facebook right now. Facebook Developer registration changed, updating those steps and screenshots as well, plus updating the code examples to address default change to Bootstrap4 and we no longer use .AddIdentityUI() to set it otherwise.

[wadepickett]

Fixed with PR #15838