docs: expand README with editor features, keybindings, and terrain/mod details

[vanlueckn]

Summary

• Updated CI badge URL to reflect repository rename (vanlueckn/open-train-simulator → Connected-Rails/connected-rails)
• Expanded main menu description to highlight clickable interface for line/vehicle/scenario selection
• Added keybindings for wipers (Y), AFB controls (6–8), headlights/cab light (9/0), backlighting dimmer (,/.)
• Clarified mod manager behavior and added mouse control documentation
• Added new mod content types: signal models, track types, objects, HTML displays
• Documented terrain splatting, vegetation, and track objects in world-render
• Expanded editor descriptions: route editor tools table (select, draw, place, terrain), vehicle editor cab/displays/sounds, signal editor
• Added Ctrl+Z/Y/Shift+Z undo/redo and rule checker to route editor documentation
• Documented DGM tile import and terrain view toggle in route editor
• Fixed scenario syntax example (edge: (2) → edge: EdgeId(2))
• Updated release workflow notes: signal editor now built from source
• Clarified custom property options: ts_motion now includes emissive, added _NIGHT suffix convention

Testing

• Verified all keybindings match current implementation in codebase
• Confirmed URL changes point to correct GitHub repository
• Checked that editor tool descriptions and UI flows match actual implementations
• Validated mod content type listing aligns with mod.ron structure
• Reviewed scenario syntax fix against compiler expectations
• Not run: visual inspection of rendered markdown (recommendation: preview locally before merge)