Document HTML helpers

This page mentions HTML helpers, but there don't appear to documented in any meaningful way. What are HTML helpers? Which built-in HTML helpers are available? How can they be customized? Etc.

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#### Document Details

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* ID: d2e47828-6da8-2323-053f-dcbf7475b1b6
* Version Independent ID: 62d4c2be-4233-64e1-2a3d-8a5e98d9d941
* Content: [Views in ASP.NET Core MVC](https://docs.microsoft.com/en-us/aspnet/core/mvc/views/overview?view=aspnetcore-6.0)
* Content Source: [aspnetcore/mvc/views/overview.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/mvc/views/overview.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-mvc**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**

Inizia da aspnetcore/mvc/views/overview.md e rivedi il riferimento esistente agli HTML helpers. Identifica gli helper integrati e le indicazioni sulla personalizzazione mancanti, quindi aggiorna la pagina in modo che i lettori possano capire cosa sono gli HTML helpers, trovare gli helper integrati disponibili e imparare a personalizzarli.

Scritto dal modello di indicizzazione a partire dal testo della issue.