feat: Allow extensionless URL HTML file access

[filzrev]

This PR is intended to add support for extensionless HTML URLs for docfx serve command.

Background
GitHub Pages support extensionless URLs. (e.g. https://dotnet.github.io/docfx/docs/basic-concepts)
And other static HTML hosting site also have similar features.

So I thought it's reasonable to support extensionless URLs with the docfx serve command.

Extensionless URL support is partially related to #2865.
It might be better to add options to output extensionless URLs for docfx internal links.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 79.28%. Comparing base (fe673ec) to head (307c9f4).
Report is 504 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Docfx.App/RunServe.cs	0.00%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10436      +/-   ##
==========================================
+ Coverage   74.31%   79.28%   +4.96%     
==========================================
  Files         536      547      +11     
  Lines       23189    23649     +460     
  Branches     4056     4065       +9     
==========================================
+ Hits        17234    18749    +1515     
+ Misses       4853     3730    -1123     
- Partials     1102     1170      +68     

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