Prevent overwriting pinned JInja2 version

during `make install-pip-package`
Update build docs instructions

docs/builddocs.rst

@@ -9,16 +9,14 @@ generated by `sphinx <http://www.sphinx-doc.org/>`_. The C++ API documentation
 is generated by `Doxygen <http://www.doxygen.nl/>`_.
 
 Documentation can be built on Ubuntu or macOS. Building documentation on Windows
-may also be possible but it is not officially tested.
+may also be possible but is not officially tested.
 
 If you're building documentation on a computer without a display, please use
 :ref:`headless_rendering`, otherwise the Jupyter tutorials will fail to execute.
 
-Prerequisites
--------------
 
-1. Install system dependencies
-``````````````````````````````
+Install system dependencies
+---------------------------
 
 **Ubuntu**
 
@@ -34,23 +32,44 @@ First, install a TeX distribution such as `MacTeX <http://www.tug.org/mactex/>`_
 
     brew install ghostscript pandoc doxygen
 
-2. Install Python dependencies
-``````````````````````````````
+
+Building C++ documentation
+--------------------------
+
+If you only want to build C++ API documentation, clone the Open3D repo and run
+doxygen.
 
 .. code-block:: bash
 
-    pip install -r docs/requirements.txt
+   git clone https://github.com/isl-org/open3d
+   cd open3d/docs
+   doxygen Doxyfile.in
+
+Start browsing the generated C++ API documentation from the file
+``docs/doxygen/html/index.html``. Read on if you want to build the full
+documentation (including Python API and tutorials).
 
-Build
------
 
-First, clone and build Open3D from source and install the Open3D Python package.
-Visit :ref:`compilation` for details. After configuration, install the Open3D
-Python package with:
+Install or Build Open3D
+-----------------------
 
 .. code-block:: bash
 
-    make install-pip-package -j$(nproc)
+    pip install open3d
+
+To instead build Open3D from source, see :ref:`compilation`.
+
+
+Install Python dependencies
+---------------------------
+
+.. code-block:: bash
+
+    pip install -r docs/requirements.txt
+
+Build docs
+----------
+
 
 .. code-block:: bash
 

util/ci_utils.sh

@@ -70,9 +70,9 @@ install_python_dependencies() {
     if [ "$BUILD_TENSORFLOW_OPS" == "ON" ]; then
         # TF happily installs both CPU and GPU versions at the same time, so remove the other
         python -m pip uninstall --yes "$TF_ARCH_DISABLE_NAME"
-        python -m pip install -U "$TF_ARCH_NAME"=="$TENSORFLOW_VER"    # ML/requirements-tensorflow.txt
+        python -m pip install -U "$TF_ARCH_NAME"=="$TENSORFLOW_VER" # ML/requirements-tensorflow.txt
     fi
-    if [ "$BUILD_PYTORCH_OPS" == "ON" ]; then    # ML/requirements-torch.txt
+    if [ "$BUILD_PYTORCH_OPS" == "ON" ]; then # ML/requirements-torch.txt
         if [[ "$OSTYPE" == "linux-gnu"* ]]; then
             python -m pip install -U "${TORCH_GLNX}" -f "$TORCH_REPO_URL" tensorboard
 
@@ -414,9 +414,10 @@ build_docs() {
         -DBUILD_WEBRTC=OFF \
         -DBUILD_JUPYTER_EXTENSION=OFF \
         ..
-    make install-pip-package -j$NPROC
+    make python-package -j$NPROC
     make -j$NPROC
     bin/GLInfo
+    export PYTHONPATH="$PYTHONPATH:$PWD/lib/python_package"
     python -c "from open3d import *; import open3d; print(open3d)"
     cd ../docs # To Open3D/docs
     python make_docs.py $DOC_ARGS --clean_notebooks --execute_notebooks=always --py_api_rst=never
@@ -434,7 +435,7 @@ build_docs() {
         -DBUILD_WEBRTC=ON \
         -DBUILD_JUPYTER_EXTENSION=OFF \
         ..
-    make install-pip-package -j$NPROC
+    make python-package -j$NPROC
     make -j$NPROC
     bin/GLInfo || echo "Expect failure since HEADLESS_RENDERING=OFF"
     python -c "from open3d import *; import open3d; print(open3d)"