message: doc review: comment on remove build directory
diff: @@ -27,8 +27,7 @@ pip install -r ext/gudhi-deploy/test-requirements.txt # pytorch can be painful
  In order to compile all c++ utilities, examples, benchmarks, unitary tests, and python module:
  ```bash
  cd /workdir/gudhi
  -rm -rf build
  -mkdir build
  +rm -rf build; mkdir build # /!\ any existing build folder will be removed
  cd build
  # To build all even examples and benchmarks
  cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=$CONDA_PREFIX -DWITH_GUDHI_EXAMPLE=ON -DWITH_GUDHI_BENCHMARK=ON ..
  @@ -39,8 +38,7 @@ cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=$CONDA_PREFIX -DWITH_GUDHI_
  In order to compile only python module
  ```bash
  cd /workdir/gudhi
  -rm -rf build
  -mkdir build
  +rm -rf build; mkdir build # /!\ any existing build folder will be removed
  cd build
  # To build all even examples and benchmarks
  cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=$CONDA_PREFIX ..
  @@ -60,8 +58,7 @@ PYTHONPATH=/workdir/gudhi/build/src/python python # or ipython, jupyter, ...
  ```bash
  cd /workdir/gudhi
  -rm -rf build
  -mkdir build
  +rm -rf build; mkdir build # /!\ any existing build folder will be removed
  cd build
  # python OFF to prevent python modules search makes cmake faster
  cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=$CONDA_PREFIX -DWITH_GUDHI_PYTHON=OFF -DUSER_VERSION_DIR=version ..
  @@ -80,8 +77,7 @@ firefox html/index.html # [optional] To display the c++ documentation. Anything
  ```bash
  cd /workdir/gudhi
  -rm -rf build
  -mkdir build
  +rm -rf build; mkdir build # /!\ any existing build folder will be removed
  cd build
  # python OFF to prevent python modules search makes cmake faster - it is the next one in user version that matters
  cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=$CONDA_PREFIX -DWITH_GUDHI_PYTHON=OFF -DUSER_VERSION_DIR=version ..
  