[Feature Request] Custom TemplateRenderer support

[filzrev]

Is your feature request related to a problem? Please describe.

I want to create custom docfx theme by using Razor C# template.
Because current Mustache based-template has limited features and hard to maintenance.

On docfx v3. There is a plan to support Razor-based template system.
https://github.com/dotnet/docfx/blob/v3/docs/designs/overview.md#razor-template-engine

And it seems some functionalities are already implemented to provide landing pages.
https://github.com/dotnet/docfx/tree/v3/src/Microsoft.Docs.Build.Specialized/template

Describe the solution you'd like
Support ITemplateRenderer based custom renderer for Docfx.App user.

For docfx.exe user.
It might needs to support following features at first.

#9058 (comment)

Eventually I think we would publish templates separately from the executable, for both the official templates and community templates.
Packaging options are open for discussion, such as NuGet (#7974), npmjs, git, OCI, etc.

Implementation consideration

Current TemplateManager related class assuming Mustache(or Liquid) based template files.
So it require some code modification.

As a minimum solution. I'm considering following code changes on local.

1. When custom plugin is loaded. It register ITemplateRenderer with supported document type. (e,g, Conceptual/LandingPage)
2. When processing model's with TemplateModelTransformer. Dynamically switch renderer based on document type.

If there are clearer way to support custom template renderer., Please let me know.

[yufeih]

Maybe we could expose an HTML render callback through Docfx.App, razor templates probably needs a code project anyway and the templates can be shared using NuGet as .NET libraries.