Add basic documentation for Polymorphic Serialization

[christianhelle]

This pull request updates the CLI option descriptions for --use-polymorphic-serialization, --use-dynamic-querystring-parameters, and --use-apizr in the README file. It also updates the file format documentation for the .refitter file to include the usePolymorphicSerialization option. These changes improve the clarity and accuracy of the CLI options and file format documentation for Refitter.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.51%. Comparing base (646b180) to head (b165b75).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #466      +/-   ##
==========================================
+ Coverage   85.95%   94.51%   +8.56%     
==========================================
  Files          50       50              
  Lines        2769     2462     -307     
==========================================
- Hits         2380     2327      -53     
+ Misses        357      108     -249     
+ Partials       32       27       -5     

Flag	Coverage Δ
unittests	94.51% <ø> (+8.56%)	⬆️

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