(MAINT) Update OSInfo schema and docs

[michaeltlombardi]

PR Summary

This change updates the Microsoft/OSInfo resource based on the work for documenting it in MicrosoftDocs/PowerShell-Docs-DSC#181

This change:

• Minimally modifies the README for grammar and formatting.
• Adds docs strings to the types in the source code.
• Adds a canonical schema for the resource, and a YAML version of the schema for easier reading and maintaining.
• Updates the resource manifest to embed the canonical schema.

PR Context

The work to define reference documentation for this resource indicated a few changes could be made to the schema definition:

1. The schema definition lacked any documentation. If the schema is used by integrating tools to enhance the authoring experience, it would be able to provide validation but not contextual help.
2. The schema definition used the multi-type ["null", "string"] where null didn't have any semantics, it just represented the absence of a value.

The canonical schema is generated from the YAML version using the following command:

$YamlPath = Resolve-Path -Path .\schemas\resources\Microsoft\OSInfo\v0.1.0\schema.yaml
| Select-Object -ExpandProperty Path
$JsonPath = $YamlPath -replace '\.yaml$', '.json'

$Schema       = Get-Content -Raw -Path $YamlPath | ConvertFrom-Yaml
$Schema.'$id' = $Schema.'$id' -replace '\.yaml$', '.json'

$Schema
| ConvertTo-Json -Depth 100
| Out-File -Encoding utf8 -Path $JsonPath -Force

The schema uses the first sentence from the documentation as the value for the description keyword for each property, replacing backticks with single quotes, as the description keyword doesn't reliably render as Markdown for editors that support hover help for JSON schemas. After every description string is the link to that property in the documentation.

The markdownDescription is a non-standard keyword supported by the JSON and YAML language servers in VS Code. When it is a non-empty string for a schema or subschema, it's rendered and used in the hover text instead of the description keyword.

The other major change in the schema is to replace the semantically incorrect multi-type of null and string with a single type of string. The JSON schema documentation states:

It's important to remember that in JSON, null isn't equivalent to something being absent.

For JSON schemas, the required keyword indicates whether a property is required. By convention, if a property isn't required and isn't intended to be semantically null, it should be omitted from the JSON representation of the object instance. Using null in a multi-type should be reserved for when a null value carries specific semantics.