This section should define "mutable", "immutable" and "readonly"

### Type of issue

Missing information

### Description

These concepts are important for developers to use the latest improvements in C#, where mutability and readonly are easier to implement than before.

### Page URL

https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/types/

### Content source URL

https://github.com/dotnet/docs/blob/main/docs/csharp/fundamentals/types/index.md

### Document Version Independent Id

9aa4f01b-a5a0-3cb1-3c22-db333c0d9f11

### Article author

@BillWagner

### Metadata

* ID: 1991c89a-9728-ea31-a999-79b3cb331372
* Service: **dotnet-csharp**
* Sub-service: **fundamentals**