[Bug] It is not generating pdf from .md files

**Describe the bug**
It is not generating pdf from .md files

**To Reproduce**
create .md files and try to generate pdf.

**Expected behavior**
Generate pdf from .md files

- OS: Windows 11
- Docfx version: [e.g. 2.77.0]

**Additional context**

```
{
"metadata": [
{
"src": [
{
"src": "../src",
"files": [
"**/*.csproj"
]
}
],
"dest": "api"
}
],
"build": {
"content": [
{
"files": [
"**/*.{md,yml}","./docs/images/**", "images/**", "./bomber-icon.png"
],
"exclude": [
"_site/**"
]
}
],
"resource": [
{
"files": [
"docs/images/**", "images/**", "index.html", "Demo/**", "index.css", "Videos/**"
]
}
],
"output": "_site",
"template": [
"default"
],
"globalMetadata": {
"_appName": "Bomber Kit",
"_appTitle": "Bomber Kit",
"_appLogoPath": "Images/bomber-icon.png",
"_appFooter": ".inheritedMembers { display: none; }",
"_enableSearch": true,
"homepage": "index.md",
"pdf": true
}
}
}
```


Run docfx pdf.


./doc/*.md files will be ignored.