[Bug] api/index.md link in toc.yml breaks API docs

[mikernet]

Describe the bug
I prefer my table of contents "API documentation" link to point the page where it shows all the main namespaces instead of going to the first type, i.e. I prefer it to go here:

The problem is that setting up the toc.yml file like this:

- name: API Documentation
  href: api/

Goes directly to the first type in the first namespace instead of the view with all the namespaces....so, this should work instead:

- name: API Documentation
  href: api/index.md

But alas, it does not - when you click the link, it shows blank content and all of the API docs are gone. None of the API docs work, even if you link directly to a member. It breaks the entire site for some reason. Note that api/index.md links within markdown documents work just fine.

This works, but it causes warning InvalidFileLink: Invalid file link:(~/api/index.html) during docfx build:

- name: API Documentation
  href: api/index.html

[filzrev]

What happens if a topicHref is specified as followings?

- name: API Documentation
  href: api/
  topicHref: api/index.md