doc: remove (outdated) primer documents

The ACRN primer has be replaced by an updated set of ACRN High-Level Design
documents.  Remove the primer (and images), and replace a link to the
graphics mediation section (from API documentation) to the corresponding
section in the APL GVT-g document.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

Author: dbkinder

doc/developer-guides/contribute_guidelines.rst

@@ -122,8 +122,7 @@ devicemodel, and doc folders) use::
 
     git clone https://github.com/projectacrn/acrn-hypervisor
 
-The project ACRN directory structure is described in the :ref:`primer`
-document. In addition to the ACRN hypervisor and device model itself,
+In addition to the ACRN hypervisor and device model itself,
 you'll also find the sources for technical documentation available from
 the `ACRN documentation site`_.  All of these are available for
 developers to contribute to and enhance.

doc/developer-guides/hld/hld-APL_GVT-g.rst

@@ -186,6 +186,8 @@ Single Root IO Virtualization (SR-IOV) implements I/O virtualization
 directly on a device. Multiple Virtual Functions (VFs) are implemented,
 with each VF directly assignable to a VM.
 
+.. _Graphic_mediation:
+
 Mediated Pass-Through
 *********************