docs: add Transfer Save Files and Cheat Codes pages Fixes two broken links on the Home page. Both pages were referenced in the User Guide section but had no corresponding .md file. - Transfer-Save-Files.md: covers battery save locations, which cores changed for GBA/SMS/Game Gear/NES, how to rename/copy saves when switching cores, and what to do if a save won't load. - Cheat-Codes.md: covers supported code formats per system, how to enter and toggle codes via the HUD cheat manager, and where to find codes online. (assisted by Claude) Made-with: Cursor