Add a doc for using ccache, cmake, ninja, clang, clangd, and mold

[scarf005]

without ccache and mold, rebuilds are considerably slower. using mold makes link time near instant (ninja clean && ninja 6.7s -> 1.2s). faster builds means faster development.

Current cmake flag in use

    cmake .. \
        -G Ninja \
        -DCMAKE_C_COMPILER=clang \
        -DCMAKE_CXX_COMPILER=clang++ \
        -DCMAKE_CXX_FLAGS="-fuse-ld=mold -Wno-unused-command-line-argument" \
        -DUSE_HOME_DIR=OFF \
        -DCMAKE_BUILD_TYPE=RelWithDebInfo \
        -DCURSES=OFF \
        -DTILES=ON \
        -DSOUND=ON \
        -DCMAKE_EXPORT_COMPILE_COMMANDS=ON

Some related Flags

CMAKE_EXPORT_COMPILE_COMMANDS: used for clangd
CMAKE_CXX_FLAGS: see https://stackoverflow.com/a/64174822/13503626