[Feature Request] Custom TemplateRenderer support

**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.

https://github.com/dotnet/docfx/issues/9058#issuecomment-1671314416
> 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 (https://github.com/dotnet/docfx/issues/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.