Add preview branch preparation documentation

[Copilot]

• Explore repository structure and understand the context
• Create preview branch preparation documentation
• Add section about changing dotnet/dotnet subscription channel
• Include steps for workload insertion preparation
• Add documentation links to README
• Add section for updating MAUI package IDs
• Add section for updating VS manifest IDs for mono and emsdk workloads
• Add --source-repo and --target-branch parameters to get-subscriptions command
• Clarify that major version previews ship from main branch
• Update MAUI package ID examples to use RC 1 to RC 2
• Update VS manifest section with specific file names
• Add note about dotnet-dotnet subscription source for RC and servicing
• Update channel name format with SDK and version details
• Remove VS team coordination references
• Review and validate documentation

Original prompt

Please create a document that outlines the process we go through during previews to prepare the branches for a workloads insertion.

• Change the channel for the dotnet/dotnet subscription

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[marcpopMSFT]

@copilot Besides updating the channel, two more updates are needed:

1. Update the package ID for all of the maui packages in this repo
2. Update the vsman id and external.vsmanproj in VS for the mono and emsdk workloads

[Copilot]

@copilot Besides updating the channel, two more updates are needed:

1. Update the package ID for all of the maui packages in this repo
   ...

Added both items to the documentation:

1. Section 3 - Update Package IDs for MAUI Packages: Detailed steps for updating the package ID suffixes in eng/Versions.props (e.g., Manifest100100rc2 → Manifest100100rc3)

2. Section 4 - Update VS Manifest IDs for Mono and Emsdk Workloads: Instructions for updating VSMAN IDs and external.vsmanproj references for both workloads

Commit: 0caff76

[marcpopMSFT]

I think I found all of the issues. Did a couple of reviews and asked it to fix all the things I found.