docs: Move the gallium driver documentation to the top level.

I actually had never found these, buried under Developer Topics -> Gallium
-> Drivers.  Given that driver documentation contains not just gallium
driver documentation but also end-user information, bring it to a much
more prominent location between User Topics and Developer Topics at the
top level.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7174>
diff --git a/docs/contents.rst b/docs/contents.rst
index f7fd50d..8b93dbe 100644
--- a/docs/contents.rst
+++ b/docs/contents.rst
@@ -49,6 +49,16 @@
 
 .. toctree::
    :maxdepth: 1
+   :caption: Drivers
+   :hidden:
+
+   drivers/freedreno
+   drivers/llvmpipe
+   drivers/openswr
+   drivers/zink
+
+.. toctree::
+   :maxdepth: 1
    :caption: Developer Topics
    :hidden: