[Feature Request] docfx modern template support for partial template

Hello,

I need to customize the modern template to be able to integrate Algolia search into my documentation
Currently to get there, I need to copy the entire template layout file and need to modify contents : https://github.com/dotnet/docfx/discussions/9668

Unfortunately, by doing this, I'm not taking advantage of any changes and corrections made to the template by docfx.

It would be great to be able to define files that will be concatenated with existing content:
- partials/head.tmpl.partial
- partials/body.tmpl.partial: