Overhaul migration guide: phased playbook + 7 deep-dive articles

[SufficientDaikon]

PR Summary

Restructure the Windows PowerShell 5.1 → PowerShell 7 migration story from
a feature overview into an actionable, phased migration playbook with
7 new deep-dive articles covering the top community pain points.

Addresses #12948.

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes
  it proposes.
• Summary: This PR's summary describes the scope and intent of the
  change.
• Contributor's Guide: I have read the contributor's guide.
• Style: This PR adheres to the style guide.

---

At a glance

The existing migration doc (Migrating-from-Windows-PowerShell-51-to-PowerShell-7.md)
tells readers what PowerShell 7 offers but not how to migrate.
The differences doc is a thorough technical reference but is not
actionable. This PR creates a hub-and-spoke model:

• Rewrite the existing migration doc as a phased guide
  (assess → install → migrate → validate → rollback)
• Add 7 deep-dive articles covering real pain points
  backed by community issue data
• Link to the existing differences doc for technical reference
  (no duplication)

Pain points covered (with issue evidence)

Pain point	Evidence	Article
Module compatibility confusion	PowerShell#4062 (63 comments), #15428 (33 comments)	module-compatibility-strategy.md
PSModulePath + OneDrive conflicts	PowerShell#15552 (109 comments)	module-compatibility-strategy.md
Encoding changes (UTF-8 NoBOM)	PowerShell#1995 (201 comments), #707 (22 comments)	encoding-changes.md
Remoting endpoint defaults to WinPS	PowerShell#11616 (32 comments)	Hub article (Phase 4)
Air-gapped network slow startup	PowerShell#10983 (58 comments)	enterprise-deployment.md
Profile path differences	PowerShell#4682	profile-migration.md
Removed cmdlets (WMI, Workflow, EventLog)	Referenced in differences doc	script-compatibility-audit.md
PSScheduledJob removal	Referenced in differences doc	scheduled-tasks-automation.md

Files changed

#	File	Status	Description
1	Migrating-from-Windows-PowerShell-51-to-PowerShell-7.md	Rewritten	Feature overview → phased migration playbook
2	migration/script-compatibility-audit.md	New	PSScriptAnalyzer scanning, removed cmdlet map, .NET changes, cross-version patterns
3	migration/module-compatibility-strategy.md	New	Module testing, compatibility layer (bugs + workarounds), PSModulePath conflicts
4	migration/encoding-changes.md	New	UTF-8 NoBOM defaults, 3 migration strategies, BOM considerations, diagnostic scripts
5	migration/profile-migration.md	New	Path differences, 3 sharing strategies, OneDrive workaround, common pitfalls
6	migration/enterprise-deployment.md	New	MSI/SCCM/Intune/winget, air-gapped networks, GPO, CI/CD pipeline updates
7	migration/scheduled-tasks-automation.md	New	PSScheduledJob replacement, Task Scheduler args, audit + bulk update scripts
8	migration/testing-and-rollback.md	New	Pester compat, validation checklist, rollback procedures, coexistence timeline
9	differences-from-windows-powershell.md	Updated	Added TIP box linking to migration guide
10	module-compatibility.md	Updated	Added Windows module compat table + link to deep-dive
11	overview.yml	Updated	Added "Migration deep dives" card to landing page
12	toc.yml	Updated	Added migration subsection with all 7 articles

Total: 12 files, +2,209/-246 lines

Design principles

1. Hub-and-spoke: migration hub links to deep-dives; never duplicates
   the differences doc
2. Actionable: every section has concrete steps, scripts, or checklists
3. Issue-backed: every pain point references real GitHub issues
4. Style-compliant: passes markdownlint with repo config (100-char
   lines, ATX headers, backtick fences, dash lists, proper name
   capitalization)

Scope and limitations

In scope

Windows-focused migration from WinPS 5.1 to PS 7. All content targets Windows 10, Windows 11, and Windows Server 2016+.

Out of scope

Cross-platform migration (Linux/macOS) — covered by the existing unix-support.md. DSC migration — extensive enough for a separate PR. PowerShell 5.1 end-of-life policy — owned by Microsoft product team.

Intentionally unchanged

differences-from-windows-powershell.md is left as-is except for a TIP box cross-link. It serves as the canonical technical reference; deep-dive articles link into it by section anchor rather than duplicating content.

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[learn-build-service-prod]

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[learn-build-service-prod]

Learn Build status updates of commit 4a72435:

💡 Validation status: suggestions

File	Status	Preview URL	Details
reference/docs-conceptual/whats-new/migration/testing-and-rollback.md	💡Suggestion	View (>=powershell-5.1)	Details
reference/docs-conceptual/toc.yml	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/differences-from-windows-powershell.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/Migrating-from-Windows-PowerShell-51-to-PowerShell-7.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/migration/encoding-changes.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/migration/enterprise-deployment.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/migration/module-compatibility-strategy.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/migration/profile-migration.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/migration/scheduled-tasks-automation.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/migration/script-compatibility-audit.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/module-compatibility.md	✅Succeeded	View (>=powershell-5.1)
reference/docs-conceptual/whats-new/overview.yml	✅Succeeded	View (>=powershell-5.1)

reference/docs-conceptual/whats-new/migration/testing-and-rollback.md

• Line 0, Column 0: [Suggestion: filename-contains-stop-word - See documentation] Stop words like 'a', 'or', 'and', 'the', and 'in' are discouraged in filenames.

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