Better document how to add customizations to the Windows TitleBar

### Type of issue

Missing information

### Description

Currently the TitleBar can be customized visually through WinUI Templates. We get a number of issues from people around how to apply these customizations. It'd be useful to have docs (and possibly some samples) around how to use these APIs.

https://github.com/dotnet/maui/issues/18773#issuecomment-1815470830

You can also customize the drag areas if you have interactive controls
https://github.com/dotnet/maui/pull/14517/files#diff-82efa837a0cdb653a6f7dd5df4159c10a7afd3f3a08bd21d0562e4c925763deaR13-R39

### Page URL

https://learn.microsoft.com/en-us/dotnet/maui/fundamentals/windows?view=net-maui-8.0

### Content source URL

https://github.com/dotnet/docs-maui/blob/main/docs/fundamentals/windows.md

### Document Version Independent Id

c7dc34b7-8355-cff9-6754-c9fdefade29a

### Article author

@davidbritch

### Metadata

* ID: c7dc34b7-8355-cff9-6754-c9fdefade29a
* Service: **dotnet-mobile**
* Sub-service: **dotnet-maui**