Proposal: Documentation xref file for WinUI 3 / WinAppSDK

# Proposal: Documentation xref file for WinUI 3 / WinAppSDK

## Summary

There does not appear to be a WinUI 3 xref service or xref map json file that can be used for documentation (or at least I can't find it), which means references to any of those types can't be cross-linked with MS docs and their full type names get output by `docfx` instead of more succinct clickable links:


This is the only time I have run into this. Even the community toolkit has proper xrefs. The community toolkit's docs on learn.ms.com also don't cross-link correctly with WinAppSDK so it seems that this is indeed an issue.

No file or test is named. Start by investigating how the documentation build invokes docfx and whether an existing xref map or service configuration is present. Compare the WinUI 3/WinAppSDK references with the Community Toolkit example mentioned in the issue; done means those references produce succinct clickable links.

Written by the indexing model from the issue text.