chore: update slicer-2 logo

[liamnwhite1]

This pull request updates the application's icon usage by renaming and replacing the old Slicer-2 icon files with consistently named new ones (slicer-2_logo.png and .ico). It updates all references across the codebase, resources, documentation, and installer scripts to use the new icon names and paths. Additionally, minor improvements are made to documentation links and formatting.

Icon Standardization and Replacement:

• Replaced all references to slicer2.png and slicer2.ico with slicer-2_logo.png and slicer-2_logo.ico throughout the codebase, including UI components, window icons, and action icons (src/widgets/gcode_info_control.cpp, src/widgets/settings/setting_bar.cpp, src/widgets/settings/setting_tab.cpp, src/windows/about.cpp, src/windows/external_file_window.cpp, src/windows/flowratecalc.cpp, src/windows/gcode_export.cpp, src/windows/layer_times_window.cpp, src/windows/main_window.cpp, include/graphics/objects/cube_object.h) [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11].
• Updated resource file icons.qrc to remove old icon files and include the new ones (resources/icons/icons.qrc).
• Deleted old icon files and added new ones in the resources/icons directory (resources/icons/slicer2.png, resources/icons/slicer2.ico, resources/icons/slicer-2_logo.png, resources/icons/slicer-2_logo.ico) [1] [2] [3] [4].

Installer and Build Script Updates:

• Modified installer scripts to use the new icon filenames for display and shortcuts (scripts/installer.nsi) [1] [2] [3].

Documentation and User Guide Improvements:

• Updated image and QR code paths in README.md to use the new icon location, and improved the user guide link and contact email formatting (README.md) [1] [2] [3].