Fix tasks skipping if PlatyPS is not available

[johlju]

Pull Request (PR) description

• Generate_Markdown_For_Public_Commands.build
  • Now the task will skip if PlatyPS is not available.
    Generate_External_Help_File_For_Public_Commands
  • Now the task will skip if PlatyPS is not available.

This Pull Request (PR) fixes the following issues

None. Helps with PR SynEdgy/Sampler.DscPipeline#33.

Task list

• Added an entry to the change log under the Unreleased section of the file CHANGELOG.md.
  Entry should say what was changed and how that affects users (if applicable), and
  reference the issue being resolved (if applicable).
• Documentation added/updated in README.md.
• Comment-based help added/updated.
• Localization strings added/updated in all localization files as appropriate.
• Examples appropriately added/updated.
• Unit tests added/updated. See DSC Community Testing Guidelines.
• Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• New/changed code adheres to DSC Community Style Guidelines.

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This change is 

[johlju]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @johlju)