Fix #9990 header and footer template files

[Foitn]

Add reading of file, instead of only allowing the template to be added to the toc.yml

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.86%. Comparing base (fe673ec) to head (77d07e4). Report is 204 commits behind head on main.

Files	Patch %	Lines
src/Docfx.App/PdfBuilder.cs	80.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10012      +/-   ##
==========================================
+ Coverage   74.31%   78.86%   +4.54%     
==========================================
  Files         536      540       +4     
  Lines       23189    23482     +293     
  Branches     4056     4067      +11     
==========================================
+ Hits        17234    18518    +1284     
+ Misses       4853     3822    -1031     
- Partials     1102     1142      +40     

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[yufeih]

Close in favor of #10099