update and expand the description of building the LAMMPS documentation

Build the LAMMPS documentation

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**CMake variable**\ :

.. code-block:: bash

The LAMMPS manual is written in `reStructuredText <rst_>`_ format which

can be translated to different output format using the `Sphinx <sphinx_>`_

document generator tool.  Currently the translation to HTML and PDF (via

LaTeX) are supported.  For that to work a Python 3 interpreter and

internet access is required.  For the documentation build a python

based virtual environment is set up in the folder doc/docenv and various

python packages are installed into that virtual environment via the pip

tool.  The actual translation is then done via make commands.

   -D BUILD_DOC=value       # yes or no (default)

This will create the HTML doc pages within the CMake build directory.

The reason to do this is if you want to "install" LAMMPS on a system

after the CMake build via "make install", and include the doc pages in

the install.

.. _rst: https://docutils.readthedocs.io/en/sphinx-docs/user/rst/quickstart.html

.. _sphinx: https://sphinx-doc.org

**Traditional make**\ :

**Documentation make option**\ :

The following make commands can be issued in the doc folder of the

LAMMPS source distribution.

.. code-block:: bash

   cd lammps/doc

   make html       # html doc pages

   make pdf        # single Manual.pdf file

  make html         # create HTML doc pages in html directory

  make pdf          # create Developer.pdf and Manual.pdf in this directory

  make fetch        # fetch HTML and PDF files from LAMMPS web site

  make clean        # remove all intermediate files

  make clean-all    # reset the entire doc build environment

  make anchor_check # scan for duplicate anchor labels

  make style_check  # check for complete and consistent style lists

  make spelling     # spell-check the manual

This will create a lammps/doc/html dir with the HTML doc pages so that

you can browse them locally on your system.  Type "make" from the

lammps/doc dir to see other options.

Thus "make html" will create a "doc/html" directory with the HTML format

manual pages so that you can browse them with a web browser locally on

your system.

.. note::

   `download page <http://lammps.sandia.gov/download.html>`_.

**CMake build option**\ :

It is also possible to create the HTML version of the manual within

the :doc:`CMake build directory <Build_cmake>`.  The reason for this

option is to include the installation of the HTML manual pages into

the "install" step when installing LAMMPS after the CMake build via

"make install".

.. code-block:: bash

   -D BUILD_DOC=value       # yes or no (default)

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