Add Gallium docs.

In Sphinx/ReST format.
diff --git a/src/gallium/docs/source/cso/sampler.rst b/src/gallium/docs/source/cso/sampler.rst
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+Sampler
+=======
+
+Texture units have many options for selecting texels from loaded textures;
+this state controls an individual texture unit's texel-sampling settings.
+
+Members
+-------
+
+XXX