Support for assets in a samples/docs

[niels9001]

Problem Statement

Would be nice to include an Assets folder in the sample project template so you can refer to local images in a sample or doc.

[michael-hawker]

We should make sure we can include any type of asset for samples as maybe there's a .json file or something that wants to be used for backing data.

But at least for images at the start, we should make sure they can be referenced in XAML samples (even if it needs the assembly name or something like this):

<Image Source="ms-appx://ProjectTemplateExperiment.Samples/FolderInSamplesProject/MyImageFile.png"/>
<!-- Or ideally short-hand: -->
<Image Source="ms-appx:///FolderInSamplesProject/MyImageFile.png"/>
<Image Source="FolderInSamplesProject/MyImageFile.png"/>

But also easily in the Markdown as well:

![Short Description](FolderInSamplesProject/MyImageFile.png "Longer Description text.")

[michael-hawker]

Was needing unoplatform/uno#2502 which should now be in latest uno nightly build.

This should allow us to load rive image and animation for Rive sample in WASM as well as tidy up all our C#/XAML sample file shenanigans.

[michael-hawker]

@Arlodotexe we should also be able to just include images as Content or something and reference in a Markdown file:

![Example of foo](images/foo.png)

Don't think this works currently? Not sure where it's looking or what settings I need, will fiddle a bit. Related to #36

[michael-hawker]

May be dependent on just fixing #18?

I can load images I copy over to the head with Content, but not sure how to load them from the Samples assembly...