Document our various CI Processes

[Rosuavio]

We have a fair amount of code present primarily to run as part of the CI pipeline. Documentation on how this code works as well on how to use it could serve to help contributors in using and improving the CI pipeline.

Some key things to document.

• How the CI pipeline is triggered and configured.
• The Cake build tool.
• The headers situation
• The build/build.ps1 script.
• All the commands to invoke particular parts of the CI pipeline.
  • Tests/checks
  • build steps
• How to add to the CI pipeline.

[Rosuavio]

Part of the tests that needs documenting is the incoming smoke tests. CommunityToolkit/WindowsCommunityToolkit#3450

[Rosuavio]

How the automated UI tests work, and how to add new ones should also be documented. CommunityToolkit/WindowsCommunityToolkit#3482

[michael-hawker]

We should add how our smoke tests work see CommunityToolkit/WindowsCommunityToolkit#3448 (comment)

Including how to grab stats on artifact size from published artifacts:
https://dev.azure.com/dotnet/WindowsCommunityToolkit/_build/results?buildId=41520&view=artifacts&type=publishedArtifacts