docs: add some very basic documentation about zink

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5967>

diff --git a/docs/gallium/drivers/zink.rst b/docs/gallium/drivers/zink.rst
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+Zink
+====
+
+Overview
+--------
+
+The Zink driver is a Gallium driver that emits Vulkan API calls instead
+of targeting a specific GPU architecture. This can be used to get full
+desktop OpenGL support on devices that only support Vulkan.
+
+Debugging
+---------
+
+There's a few tools that are useful for debugging Zink, like this environment
+variable:
+
+.. envvar:: ZINK_DEBUG <flags> ("")
+
+``nir``
+   Print the NIR form of all shaders to stderr.
+``spirv``
+   Write the binary SPIR-V form of all compiled shaders to a file in the
+   current directory, and print a message with the filename to stderr.
+``tgsi``
+   Print the TGSI form of TGSI shaders to stderr.
+
+Vulkan Validation Layers
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Another useful tool for debugging is the `Vulkan Validation Layers
+<https://github.com/KhronosGroup/Vulkan-ValidationLayers/blob/master/README.md>`_.
+
+The validation layers effectively insert extra checking between Zink and the
+Vulkan driver, pointing out incorrect usage of the Vulkan API. The layers can
+be enabled by setting the environment variable :envvar:`VK_INSTANCE_LAYERS` to
+"VK_LAYER_KHRONOS_validation". You can read more about the Validation Layers
+in the link above.
+
+IRC
+---
+
+In order to make things a bit easier to follow, we have decided to create our
+own IRC channel. If you're interested in contributing, or have any technical
+questions, don't hesitate to visit `#zink on FreeNode
+<irc://irc.freenode.net/zink>`_ and say hi!