Improve the look of new version release notes (#12229)

# Description
Adds md4c to parse release notes to HTML

# Screenshots/Recordings/Graphs

<img width="691" height="574" alt="image" src="https://github.com/user-attachments/assets/1ecd9bf4-6e26-4702-880e-c6f585c145ce" />


## 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