[ADD] point_of_sale: posbox documentation

This is a big update of the posbox documentation in
addons/hw_proxy/doc. The main reason for this big change is v13 of the
posbox image which is vastly different from all the images that came
before it. On top of that, the documentation was also further updated to
look a bit better when rendered to HTML instead of PDF, which is what we
used before.

point_of_sale.rst

@@ -0,0 +1,10 @@
+:banner: banners/pos.jpg
+
+=============
+Point of Sale
+=============
+
+.. toctree::
+   :titlesonly:
+
+   point_of_sale/posbox