move 'dune mental model' to the top of the section

igned-off-by: sabine <[email protected]>

Author: sabine

doc/explanation/index.rst

@@ -7,11 +7,11 @@ the rest of the OCaml ecosystem.
 .. toctree::
    :maxdepth: 1
 
+   mental-model
    scopes
    preprocessing
    ocaml-ecosystem
    package-management
    opam-integration
    bootstrap
-   mental-model
    tour/index