[docs] Document how to install sphinx and recommonmark on Ubuntu

llvm/docs/SphinxQuickstartTemplate.rst

 `recommonmark <https://recommonmark.readthedocs.io/en/latest/>`_ extension.
 
 On Debian you can install these with:
 
 .. code-block:: console
 
    sudo apt install -y sphinx-doc python-recommonmark-doc
 
+On Ubuntu use pip to get an up-to-date version of recommonmark:
+
+.. code-block:: console
+
+   sudo pip install sphinx recommonmark
+
 Then run cmake to build the documentation inside the ``llvm-project`` checkout:
 
 .. code-block:: console
 
    mkdir build
    cd build
    cmake -DLLVM_ENABLE_SPHINX=On ../llvm
    cmake --build . --target docs-llvm-html
 
 In case you already have the Cmake build set up and want to reuse that, 
 just set the CMake variable ``LLVM_ENABLE_SPHINX=On``.
 
 After that you find the generated documentation in ``build/docs/html``
 folder.