Add YAML metadata to front matter to identify preview modules

[theJasonHelmick]

Summary of the new feature / enhancement

In reference to: https://dev.azure.com/ceapex/Engineering/_workitems/edit/404408

We need to define a meta data key that PlatyPS can render to yaml -- perhaps maml. Then OPS would use this information to render the appropriate preview disclaimers on the web pages.

Proposed technical implementation details (optional)

No response

[sdwheeler]

How do you want PlatyPS to determine that the module is prerelease (-AsPreview switch, 0.x version, sematic version with prerelease)?
Which files should have the metadata (cmdlets, module, both)?
What should the metadata be (eg ispreview: true|false)?
Separate cmdlet to add arbitrary metadata?
What should OPS do with this metadata when rendering?

[sdwheeler]

@mikefrobbins We need to talk with Damien about how this will be used, where the metadata belongs (cmdlet, module, etc), and how it should be rendered on the Docs site.

@adityapatwardhan we need to settle on a schema for this feature.