Update and reorganize Python docs

doc/src/Python_trouble.rst

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+Troubleshooting
+***************
+
+Testing if Python can launch LAMMPS
+===================================
+
+To test if LAMMPS is callable from Python, launch Python interactively
+and type:
+
+.. code-block:: python
+
+   >>> from lammps import lammps
+   >>> lmp = lammps()
+
+If you get no errors, you're ready to use LAMMPS from Python.  If the
+second command fails, the most common error to see is
+
+.. code-block:: bash
+
+   OSError: Could not load LAMMPS dynamic library
+
+which means Python was unable to load the LAMMPS shared library.  This
+typically occurs if the system can't find the LAMMPS shared library or
+one of the auxiliary shared libraries it depends on, or if something
+about the library is incompatible with your Python.  The error message
+should give you an indication of what went wrong.
+
+If your shared library uses a suffix, such as ``liblammps_mpi.so``, change
+the constructor call as follows (see :ref:`python_create_lammps` for more details):
+
+.. code-block:: python
+
+   >>> lmp = lammps(name='mpi')
+
+You can also test the load directly in Python as follows, without
+first importing from the lammps.py file:
+
+.. code-block:: python
+
+   >>> from ctypes import CDLL
+   >>> CDLL("liblammps.so")
+
+If an error occurs, carefully go through the steps in :ref:`python_install_guides` and on the
+:doc:`Build_basics <Build_basics>` doc page about building a shared library.