Document how modern ArPow PVP works, plus context

[dagood]

There's an old doc with the initial design for PVP:
https://github.com/dotnet/source-build/blob/main/Documentation/auto-dependency-flow/api.md#file-format-package-version-props

I think ArPow needs a new doc that requires less context to understand, and adds new things that have come along since then. I think these would be useful:

• PackageVersion vs. Version suffix?
• What is the meaning of the RefOnly prefix in eng/Versions.props? (Or any other prefix?)
• What are CurrentSourceBuiltPackageVersions.props and PreviouslySourceBuiltPackageVersions.props about?
• What is the connection to Maestro++/BAR/Darc upgrades and eng/Version.Details.xml?

[dagood]

I wrote a description of PVP as I know it at dotnet/roslyn#57277 (comment), but it would be good to have a nice doc ready for this.

[MichaelSimons]

[Triage] This should be done as part of the per repo pvp work. CC @oleksandr-didyk

[oleksandr-didyk]

[Triage] This should be done as part of the per repo pvp work. CC @oleksandr-didyk

Reflected this in the original PvP issue for better tracking

[MichaelSimons]

This is currently documented in https://github.com/dotnet/source-build/blob/main/Documentation/package-dependency-flow.md.