[Housekeeping] Add SourceLink

[TheCodeTraveler]

Description of Change

This PR adds SourceLink to CommunityToolkit.Maui.Markup: https://devblogs.microsoft.com/dotnet/producing-packages-with-source-link/

Adding SourceLink enables our users to more easily debug their code.

PR Checklist

• [x] Rebased on top of main at time of PR
• [x] Changes adhere to coding standard
• [x] Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information