0.5.4 refactor and update docs

• Refactored the entire project structure by moving the contents of the rust-pixel directory to the root directory.
• Refactored the way project directories are managed: in development mode, using env::var("CARGO_MANIFEST_DIR") as the starting directory; after release, you can flexibly pass in the path via command line arguments.
• Refactored the cargo-pixel tool and set it as the binary file of rust_pixel, so you can directly install cargo-pixel via cargo install rust_pixel.
• Update README.md for new features