Add VGPU documentation (#44)

* Add VGPU documentation

* Fix typo

Co-authored-by: Mark Goddard <mark@stackhpc.com>

* Capitalisation fix

Co-authored-by: Mark Goddard <mark@stackhpc.com>

* Apply suggestions from code review

* Address comments in code review

* Update source/gpus_in_openstack.rst

Co-authored-by: Mark Goddard <mark@stackhpc.com>

* Add section on ansible inventory

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Co-authored-by: Mark Goddard <mark@stackhpc.com>

Author: jovial