docs: include meson in the toctree

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4630>
diff --git a/docs/contents.rst b/docs/contents.rst
index 6db72c4..2d44e14 100644
--- a/docs/contents.rst
+++ b/docs/contents.rst
@@ -14,7 +14,7 @@
    conform
 
 .. toctree::
-   :maxdepth: 1
+   :maxdepth: 2
    :caption: Download and Install
    :hidden: