Clarify and document rationale and methodology for release preview versioning

# Background

The 8.2 release is ongoing, with tracking of NuGet package releases in https://github.com/CommunityToolkit/Windows/issues/635

# Problem

8.2 Preview1 and 8.2 Preview2 have shipped. It's implied that at least one RC needs to ship before Stable, but this is an informed presumption at best and isn't explicitly outlined anywhere.

The rationale behind the versioning methodology for packages published to the public NuGet feed should be clearly documented.

# Solution

Investigate and clarify the rationale and methodology for moving between prerelease versions before shipping a stable package.

Once we've gathered this information, we can document it clearly for other maintainers or contributors to reference.