[Bug] No build warnings when links in markdown are missing

**Describe the bug**
When I have a text with a referenced link and that reference is missing in the link list, I do not get any build warnings about that missing ink.

**To Reproduce**
Build a docfx documentation with a markdown file `file.md` with the following content:

```markdown
# Header

This is some markdown text with a [good link] and a [missing link].

[good link]: https://github.com/dotnet/docfx/issues/8756
```

**Expected behavior**
A build waring is generated that the link `[missing link]` is missing in `file.md` on line 3.

**Actual behavior**
No build warning is generated.

**Context (please complete the following information):**
- OS: Windows
- Docfx version: 2.59.4.0