Add Gallium docs.
In Sphinx/ReST format.
diff --git a/src/gallium/docs/source/intro.rst b/src/gallium/docs/source/intro.rst
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+Introduction
+============
+
+What is Gallium?
+----------------
+
+Gallium is essentially an API for writing graphics drivers in a largely
+device-agnostic fashion. It provides several objects which encapsulate the
+core services of graphics hardware in a straightforward manner.