(GH-8706) Clarify about_Environment_Variables

[michaeltlombardi]

PR Summary

Prior to this PR, the about_Environment_Variables topic included misleading examples that would error if run in sequence. Additionally, the organization and content of the document hid some useful information in later sections, was difficult to follow, and lacked output examples.

This PR is a rewrite and restructuring of the document. The documents new flow explains, in order:

1. The most important information about environment variables in the long description
2. How to use/change environment variables
3. How to persist changes to environment variables
4. What environment variables PowerShell adds and uses

This PR resolves #8706.

PR Context

Check the boxes below to indicate the content affected by this PR.

Repository or docset configuration

• Repo documentation and configuration (.git/.github/.vscode etc.)
• Docs build files (.openpublishing.* and build scripts)
• Docset configuration (docfx.json, mapping, bread, module folder)

Conceptual documentation

• Files in docs-conceptual

Cmdlet reference & about_ topics
When changing cmdlet reference or about_ topics, the changes should be copied to all
relevant versions. Check the boxes below to indicate the versions affected by this change.

• Preview content
• Version 7.2 content
• Version 7.1 content
• Version 7.0 content
• Version 5.1 content

PR Checklist

• I have read the contributors guide and followed the style and process guidelines
• PR has a meaningful title
• PR is targeted at the staging branch
• All relevant versions updated
• Includes content related to issues and PRs - see Closing issues using keywords.
• This PR is ready to merge and is not Work in Progress. If the PR is work in progress,
  please add the prefix WIP: or [WIP] to the beginning of the title and remove the prefix when
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[opbld32]

Docs Build status updates of commit c8b4182:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-5.1)

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[sdwheeler]

LGTM - copy the changes across the versions

[opbld32]

Docs Build status updates of commit 87e6633:

✅ Validation status: passed

File	Status	Preview URL	Details
reference/5.1/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-5.1)
reference/7.0/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-7)
reference/7.1/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-7.1)
reference/7.2/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-7.2)
reference/7.3/Microsoft.PowerShell.Core/About/about_Environment_Variables.md	✅Succeeded	View (powershell-7.3)

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel