Add dedicated Markdown document icon

[Anas-Shakeel]

Summary

This PR adds a dedicated icon for Markdown documents and configures it for supported platforms.

Changes

• Added Markdown document icons:
  • md_file.ico (Windows)
  • md_file.icns (macOS)
  • md-file-icon.svg (source)
  • master_file.png (source)
• Configured file associations in deployConfig.js.

Testing

• Windows (NSIS): Custom Markdown document icon is used.
• Windows (MSI): Falls back to the application icon (expected Electron Builder limitation).
• Linux: Build succeeds. No custom document icon (expected Electron Builder limitation).
• macOS: Configuration added but untested (no macOS environment available).

All existing tests pass.

Addresses #83

[c3er]

DOCUMENT_ICON appears to be a better name. In my feeling, this would make the comment "File Association Icon" superfluous.

[c3er]

Thank you again! I should be able to look deeper into this and your other PR somewhen in the next days.

[c3er]

Thank you again for your contribution.

It works in macOS. Even with the limitations by Electron Builder, it's much better than before👍