feat(codegen):add document about dotnet decorator

[ragi-dayananda]

Signed-off-by: ragi.dayananda@docusign.com

Closes #

Changes

• Update csharp codegen document with new dotnet decorator to configure dotnet specific data type.

Screenshots

Author Checklist

• [x] Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• [ ] Vital features and changes captured in unit and/or integration tests
• [ ] Commits messages follow AP format
• [ ] Extend the documentation, if necessary
• [ ] Merging to master from fork:branchname
• [ ] Manual accessibility test performed
  • [ ] Keyboard-only access, including forms
  • [ ] Contrast at least WCAG Level A
  • [ ] Appropriate labels, alt text, and instructions

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