Add some documentation for the oct-vdw_c6 script

Nicolas Tancogne-Dejeanrequested to merge
manual/oct_vdw_c6 into main

Description

Add some documentation for the oct-vdw_c6 script

Checklist

  • I have checked that my code follows the Octopus coding standards
  • I have added tests for all the new features added in this request.

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