Conceptual docs missing for the ThemeCollection

## Describe the issue
No conceptual documentation exists for the [ThemeCollection](https://github.com/microsoft/MixedReality-GraphicsTools-Unity/blob/public/0.4.x/com.microsoft.mrtk.graphicstools.unity/Runtime/Utilities/ThemeCollection.cs)feature. 

## Feature area
No docs for the feature exist.

## Existing doc link
n/a

## Additional context
n/a
