Reorganizing documentation in preparation for p1 release

[sdwheeler]

PR Summary

Reorganized documentation in preparation for preview1 release

• Created version specific folders under ./docs
• Create a developer under ./docs
• Added cmdlet reference for Microsoft.PowerShell.PlatyPS
• Moved existing content under ./docs/*/platyPS

New folder structure:

docs
├───cmdlet-reference
│   ├───Microsoft.PowerShell.PlatyPS
│   └───platyPS
└───developer
    ├───Microsoft.PowerShell.PlatyPS
    │   └───specs
    └───platyPS
        └───presentation
            ├───css
            ├───images
            └───js