[Feature Request] Support `Overwrite Files` feature for `"outputFormat": "apiPage"`

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

When using `"outputFormat": "apiPage"` option.

It seems [Overwrite Files](https://dotnet.github.io/docfx/tutorial/intro_overwrite_files.html) is not applied.
(I've confirmed this behaviors with `docfx/samples/seed/specs` overwrite files)

And `Overwrite File` seems not works when using `"outputFormat": "markdown"` and overwrite for `conceptual` document by `uid`.

**Describe the solution you'd like**
This issue is created as reminder.

If there are users who want to using `Overwrite File` features for `ApiPage`.
It might be better to support similar `Overwrite File` feature for `apiPage`/`markdown` output format.

**Describe alternatives you've considered**
Add supported document types to [Overwrite Files document](https://dotnet.github.io/docfx/tutorial/intro_overwrite_files.html) page.