Describe the bug

docfx support build-time TOC customization by using toc.extension.js file.
But it's applied to generated html files only. Original toc.json file is not modified.

When using modern template. Generated toc.html is not used.
And toc.json file is used instead.
So build-time TOC customization is not works when using modern template.

To Reproduce

Steps to reproduce the behavior:

1. Create custom template and place custom toc.extension.js file.
2. Run docfx build command
3. Confirm generated toc.html and toc.json content.

Expected behavior

TOC customization with toc.extension.js works same as default template when using modern template.

Context (please complete the following information):

• OS: Windows
• Docfx version: [e.g. 2.76.0]