Improve the look of new version release notes (#12231)

# Description
Adds md4c to parse release notes to HTML

# Screenshots/Recordings/Graphs
<img width="695" height="580" alt="image" src="https://github.com/user-attachments/assets/8eaf2874-34f8-479a-b38c-2f9754297299" />

## Tests
Tested on Windows

src/slic3r/GUI/ReleaseNote.hpp

@@ -107,6 +107,7 @@ public:
     Button*           m_button_download;
     Button*           m_button_cancel;
     std::string       url_line;
+    std::string       html_source;
 };
 
 class SecondaryCheckDialog : public DPIFrame