docs(maskeditor): rewrite EN mask editor docs with tools, layers, shortcuts, and settings

[lin-bot23]

Summary

Rewrites the Mask Editor documentation to reflect the recent frontend updates.

Changes

• 3 ways to open: context menu, image preview button, selection toolbar
• 5 tools: Mask Pen, Paint Pen, Eraser, Paint Bucket, Color Select (with all parameters)
• TopBar actions: Undo/Redo, Rotate, Mirror, Invert, Clear, Save/Cancel
• Layer management: Mask/Paint/Base Image layers with blend modes
• Brush settings: Shape, Color, Thickness, Opacity, Hardness, Step Size
• Shortcuts: Space+Drag pan, Ctrl+Scroll zoom, Ctrl+Z undo, Alt+RMB brush adjust
• Global settings: Brush adjustment speed, dominant axis lock
• Graph Mode + App Mode support
• 3 TODO: INSERT SCREENSHOT placeholders for updated screenshots

Screenshots

Screenshots to be added by @daxiong in a follow-up commit.