Resolve API docs build issues

[david-poindexter]

The following have been done to as a part of a major effort to resolve the API documentation build/deploy issues:

• Convert custom plugins project from .NET Framework 4.5.2 to .NET Core 6.0.
• Add new DocAsCode packages to plugins project that are now required.
• Update Nuke build to use docfx instead of docfx.console.
• Resolve issue with metadata within docfx.json.
• Add /.nuke/temp to .gitignore.

[valadas]