move 'dune mental model' to the top of the section (ocaml#12645)

Signed-off-by: Sabine Schmaltz <[email protected]>
Co-authored-by: Shon Feder <[email protected]>

Author: 2 people

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