Clean-up: Remove custom content linking and SourceAssets folder for loading of Sample Code, XAML, and Markdown

Related CommunityToolkit/Labs-Windows#37, CommunityToolkit/Labs-Windows#88

_Improvement to workaround for this Uno issue_: https://github.com/unoplatform/uno/issues/8618 - If the Uno issue gets resolved, this work doesn't need to be done and we can clean-up our existing workaround in the codebase.

@Arlodotexe talked to Jerome. He suggested instead of the glob we could use an inline task to help us with our desired copying heuristic: https://github.com/unoplatform/uno/blob/43efd99f67daade2e663e317989c3ab324a2c443/src/SolutionTemplate/Uno.ProjectTemplates.Dotnet/Uno.ProjectTemplates.Dotnet.csproj#L244

That way we can use the proper globs to grab the input files from the proper locations, but then have the task make sure the extra parent/relative path parts don't matter as it'll copy it to a consistent `SourceAssets\ExperimentName.Sample\files` location which matches the metadata we generate. That way all the apps should be able to load consistently regardless of where we're building the app / grabbing the files from, right?

_Originally posted by @michael-hawker in https://github.com/CommunityToolkit/Labs-Windows/issues/130#issuecomment-1139180876_