[Docs] Fix installation of Minkowski (#2534)

Author: sunjiahao1999

docs/en/get_started.md

@@ -94,6 +94,8 @@ Note:
 
    ```shell
    conda install openblas-devel -c anaconda
+   export CPLUS_INCLUDE_PATH=CPLUS_INCLUDE_PATH:${YOUR_CONDA_ENVS_DIR}/include
+   # replace ${YOUR_CONDA_ENVS_DIR} to your anaconda environment path e.g. `/home/username/anaconda3/envs/openmmlab`.
    pip install -U git+https://github.com/NVIDIA/MinkowskiEngine -v --no-deps --install-option="--blas_include_dirs=/opt/conda/include" --install-option="--blas=openblas"
    ```
 
@@ -109,6 +111,7 @@ Note:
    ```shell
    conda install -c bioconda sparsehash
    export CPLUS_INCLUDE_PATH=CPLUS_INCLUDE_PATH:${YOUR_CONDA_ENVS_DIR}/include
+   # replace ${YOUR_CONDA_ENVS_DIR} to your anaconda environment path e.g. `/home/username/anaconda3/envs/openmmlab`.
    pip install --upgrade git+https://github.com/mit-han-lab/[email protected]
    ```
 

docs/zh_cn/get_started.md

@@ -92,6 +92,8 @@ mim install "mmdet3d>=1.1.0rc0"
 
    ```shell
    conda install openblas-devel -c anaconda
+   export CPLUS_INCLUDE_PATH=CPLUS_INCLUDE_PATH:${YOUR_CONDA_ENVS_DIR}/include
+   # replace ${YOUR_CONDA_ENVS_DIR} to your anaconda environment path e.g. `/home/username/anaconda3/envs/openmmlab`.
    pip install -U git+https://github.com/NVIDIA/MinkowskiEngine -v --no-deps --install-option="--blas_include_dirs=/opt/conda/include" --install-option="--blas=openblas"
    ```
 
@@ -107,6 +109,7 @@ mim install "mmdet3d>=1.1.0rc0"
    ```shell
    conda install -c bioconda sparsehash
    export CPLUS_INCLUDE_PATH=CPLUS_INCLUDE_PATH:${YOUR_CONDA_ENVS_DIR}/include
+    # replace ${YOUR_CONDA_ENVS_DIR} to your anaconda environment path e.g. `/home/username/anaconda3/envs/openmmlab`.
    pip install --upgrade git+https://github.com/mit-han-lab/[email protected]
    ```