remove references to .txt files for building the documentation

  style you are adding to LAMMPS. For simplicity and convenience, the

  documentation of groups of closely related commands or styles may be

  combined into a single file.  This will be one file for a single-file

  feature.  For a package, it might be several files.  These are simple

  text files with a specific markup language, that are then auto-converted

  to HTML and PDF. The tools for this conversion are included in the

  source distribution, and the translation can be as simple as doing

  feature.  For a package, it might be several files.  These are text

  files with a .rst extension using the

  :ref:`reStructuredText <https://docutils.readthedocs.io/en/sphinx-docs/user/rst/quickstart.html>`_ markup language, that are then converted to HTML

  and PDF using the :ref:`Sphinx <https://sphinx-doc.org>`_ documentation

  generator tool.   Running Sphinx with the included configuration

  requires Python 3.x.  Configuration

  settings and custom extensions for this conversion are included in

  the source distribution, and missing python packages will be

  transparently downloaded into a virtual environment via pip. Thus,

  if your local system is missing required packages, you need access

  to the internet. The translation can be as simple as doing

  "make html pdf" in the doc folder.

  Thus the documentation source files must be in the same format and

  style as other \*.txt files in the lammps/doc/src directory for similar

  commands and styles; use one or more of them as a starting point.

  A description of the markup can also be found in

  lammps/doc/utils/txt2html/README.html

  As appropriate, the text files can include links to equations

  (see doc/Eqs/\*.tex for examples, we auto-create the associated JPG

  files), or figures (see doc/JPG for examples), or even additional PDF

  files with further details (see doc/PDF for examples).  The doc page

  should also include literature citations as appropriate; see the

  bottom of doc/fix\_nh.txt for examples and the earlier part of the same

  file for how to format the cite itself.  The "Restrictions" section of

  the doc page should indicate that your command is only available if

  LAMMPS is built with the appropriate USER-MISC or USER-FOO package.

  See other user package doc files for examples of how to do this. The

  prerequisite for building the HTML format files are Python 3.x and

  virtualenv, the requirement for generating the PDF format manual

  is the `htmldoc <http://www.htmldoc.org/>`_ software. Please run at least

  "make html" and carefully inspect and proofread the resulting HTML format

  doc page before submitting your code.

  As appropriate, the text files can include inline mathematical

  expression or figures (see doc/JPG for examples).  Additional PDF

  files with further details (see doc/PDF for examples) may also be

  included.  The doc page should also include literature citations as

  appropriate; see the  bottom of doc/fix\_nh.rst for examples and

  the earlier part of the same file for how to format the cite itself.

  Citation labels must be unique across all .rst files.

  The "Restrictions" section of the doc page should indicate if

  your command is only available if LAMMPS is built with the

  appropriate USER-MISC or USER-FOO package.

  See other user package doc files for examples of how to do this.

  Please run at least "make html" and carefully inspect and proofread

  the resulting HTML format  doc page before submitting your code.

  Upon submission of a pull request, checks for error free completion

  of the HTML and PDF build will be performed and also a spell check,

  a check for correct anchors and labels, and a check for completeness

  of references all styles in their corresponding tables and lists is

  run.  In case the spell check reports false positives they can be

  added to the file doc/utils/sphinx-config/false_positives.txt

* For a new package (or even a single command) you should include one or

  more example scripts demonstrating its use.  These should run in no

  more than a couple minutes, even on a single processor, and not require