add section to docs: "Running PSI/J at your site" #299

Running PSI/J at your site
--------------------------

TODO: Pages should contain:

- A simple example ported to multiple sites showing how to configure PSI/J for
  each site with required configuration / attributes (with site-switcher?)
  (Each example should be in the test suite)
- Common errors you might encounter
- ‘If your site isn’t listed, please contact us to include it’


Running at LLNL LC
^^^^^^^^^^^^^^^^^^

Running at OLCF
^^^^^^^^^^^^^^^

Running at NERSC
^^^^^^^^^^^^^^^^

Running at ALCF
^^^^^^^^^^^^^^^