perf: Optimize pdf generation performance by reducing file I/O

[filzrev]

This PR intended to optimize PDF generation performance.

On current implementation.

1 . PrintHeaderFooterCore method is called every page numbers. And it access physical every times.
2. When header/footer is specified as string. It process try-catch operation every times.

What's changed in this PR

1. Add InvalidPathChars checks to detect pdfHeaderTemplate/pdfFooterTemplate is HTML content or not.
2. Add headerFooterTemplateCache to reduce file I/O.
3. Rename headerFooterCache to headerFooterPageCache

[codecov]

Codecov Report

Attention: Patch coverage is 13.63636% with 19 lines in your changes missing coverage. Please review.

Project coverage is 79.35%. Comparing base (fe673ec) to head (80b6806).
Report is 498 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Docfx.App/PdfBuilder.cs	13.63%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10461      +/-   ##
==========================================
+ Coverage   74.31%   79.35%   +5.03%     
==========================================
  Files         536      547      +11     
  Lines       23189    23639     +450     
  Branches     4056     4062       +6     
==========================================
+ Hits        17234    18759    +1525     
+ Misses       4853     3710    -1143     
- Partials     1102     1170      +68     

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