docs: update zellij-ps to reflect project switcher functionality

- Update package description and fix mainProgram typo
- Rewrite documentation to describe project switching, not process viewing
- Add PROJECT_FOLDERS configuration and usage examples
- Update all references across docs (README, guides, module overviews)

pkgs/zellij-ps/default.nix

@@ -34,9 +34,9 @@ with lib;
     '';
 
     meta = {
-      description = "A small project script for zellij";
+      description = "Zellij project switcher - quickly switch between project folders";
       license = lib.licenses.mit;
       platforms = platforms.unix;
-      mainProgram = "zelli-ps";
+      mainProgram = "zellij-ps";
     };
   }