docs: start with howto section

doc/src/Howto_multiple.rst

@@ -44,7 +44,7 @@ For large numbers of independent simulations, you can use
 :doc:`variables <variable>` and the :doc:`next <next>` and
 :doc:`jump <jump>` commands to loop over the same input script
 multiple times with different settings.  For example, this
-script, named in.polymer
+script, named ``in.polymer``
 
 .. code-block:: LAMMPS
 
@@ -57,7 +57,7 @@ script, named in.polymer
    next d
    jump in.polymer
 
-would run 8 simulations in different directories, using a data.polymer
+would run 8 simulations in different directories, using a ``data.polymer``
 file in each directory.  The same concept could be used to run the
 same system at 8 different temperatures, using a temperature variable
 and storing the output in different log and dump files, for example
@@ -83,10 +83,10 @@ partition of processors.  LAMMPS can be run on multiple partitions via
 the :doc:`-partition command-line switch <Run_options>`.
 
 In the last 2 examples, if LAMMPS were run on 3 partitions, the same
-scripts could be used if the "index" and "loop" variables were
+scripts could be used if the ``index`` and ``loop`` variables were
 replaced with *universe*\ -style variables, as described in the
-:doc:`variable <variable>` command.  Also, the "next t" and "next a"
-commands would need to be replaced with a single "next a t" command.
+:doc:`variable <variable>` command.  Also, the :lammps:`next t` and :lammps:`next a`
+commands would need to be replaced with a single :lammps:`next a t` command.
 With these modifications, the 8 simulations of each script would run
 on the 3 partitions one after the other until all were finished.
 Initially, 3 simulations would be started simultaneously, one on each