(GH-244) Add missing code for PSDSCv3 install

[michaeltlombardi]

PR Summary

Prior to this change, the overview for the DSCv3 documentation included a sentence about installing PowerShell DSC v3, but did not include the code block to actually install the module.

This change:

• Adds the missing code block
• Clarifies that the module is in beta
• Resolves #244
• Fixes AB#204909

  PR Checklist

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

[learn-build-service-prod[bot]]

Learn Build status updates of commit c393193:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
dsc/docs-conceptual/dsc-3.0/overview.md	:white_check_mark:Succeeded	View (dsc-3.0)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod[bot]]

Learn Build status updates of commit b556f22:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
dsc/docs-conceptual/dsc-3.0/overview.md	:white_check_mark:Succeeded	View (dsc-3.0)

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel