[Feature Request] Add feature to load coverPage/headerTemplate/footerTemplate from cofig directory based path

[filzrev]

Is your feature request related to a problem? Please describe.
Currently following file paths must be specified as relative to the root of the output directory (_site).

1. coverPage
2. headerTemplate/footerTemplate

Describe the solution you'd like

1. coverPage HTML should be loaded from configDirectory based relative path. Then fallback to outputFolder based relative path.
   It's required because it need to support cover.md -> cover.html support for backward compatibility.
2. headerTemplate/footerTemplate should loaded from configDirectory instead of outputFolder based relative path.
   On current version. It's loaded from outputFolder based relative path.
   But it seems not works property (See: [Bug] In 2.78.x PDF Footer search box appear when search is enable in docfx.json #10414). So it need to copy pdf/*.html files as resource files.