Indexes on Owned Entity Properties Should Be Covered

The topic of indexes on owned entity properties should be covered in this document.

It is is not straight forward to create indexes in this case, at least for new users, as it requires using EntityBuilder OwnsOne HasIndex as shown here https://stackoverflow.com/a/50029120/107683

---
#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: f85ea32c-8d23-d61d-6036-cd772e3e95d9
* Version Independent ID: 25dd8ac4-2521-5200-0ddf-63ac8c83787a
* Content: [Indexes - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/indexes?tabs=data-annotations)
* Content Source: [entity-framework/core/modeling/indexes.md](https://github.com/dotnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/indexes.md)
* Product: **entity-framework**
* Technology: **entity-framework-core**
* GitHub Login: @roji
* Microsoft Alias: **avickers**