Not a word about RouteDataRequestCultureProvider!

### Description

There is no word about RouteDataRequestCultureProvider, nor an example.
Please provide a full documentation and example on a scenario when pages without `{lang?}` route (*__/Index__*) are served using the default culture set in page model, and pages with `{lang?}` set (like *__/ru/Index__*) use the culture provided in the URL.

### Page URL

https://learn.microsoft.com/en-us/aspnet/core/fundamentals/localization?view=aspnetcore-8.0

### Content source URL

https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/fundamentals/localization.md

### Document ID

ad989395-3f49-4d55-521e-cf6fe56281ee

### Article author

@Rick-Anderson

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[Associated WorkItem - 199065](https://dev.azure.com/msft-skilling/Content/_workitems/edit/199065)