[Bug] Setting outputFileName has no effect when set inside fileMetadata

[glopesdev]

Describe the bug
The file metadata supports outputFileName as a way to indicate the desired file name of a specific document. For example the following YAML metadata can be indicated at the top of a README.md file to make sure the result is emitted as index.html.

---
outputFileName: index.html
---

Since outputFileName is part of file metadata, one would expect it to be possible to specify it as part of the fileMetadata section in docfx.json, similar to other metadata entries.

Unfortunately, the below doesn't seem to work:

  "build": {
    "fileMetadata": {
      "_appTitle": {
        "**/README.md": "index title"
      },
      "outputFileName": {
        "**/README.md": "index.html"
      }
    }
  }

In this case, _appTitle is applied successfully (only README.md files will have the custom title) but files will still be named README.html.

Expected behavior
All README.md files should be exported as index.html files.

Context (please complete the following information):

• OS: Windows
• Docfx version: 2.75.3

[filzrev]

I'm also facing similar problems before that documentType metadata is not applied when specified by fileMetadata.

For consistent behavior.
'documentType' and 'outputFileName' metadata should be overridable by 'fileMetadata' configurations.

Similar issues are discussed before at #1310 (comment).