Update branch snap instructions

[premun]

Context

Various folks on the .NET team have instructions on how to snap branches and re-target subscriptions when branching out for new (preview) release branches. The instructions will likely change as VMR subscriptions need to be created differently.

Goals

• [ ] Create a list of all documents, tools and scripts people use for branching
• [ ] Update the above list with new instructions

[premun]

https://github.com/dotnet/arcade/blob/main/scripts/create-preview-flow.ps1

[dkurepa]

we merged the PR adding the preview flow creation script