doc: put uncategorized pages under the best header (ocaml#7683)

Fixes ocaml#7584

This makes sure that the intermediate state we're in still makes sense
in a release. The pages in question are not 100% of the right type but
they make sense as part of the section.

Signed-off-by: Etienne Millon <me@emillon.org>

doc/index.rst

@@ -24,6 +24,15 @@ Welcome to Dune's Documentation!
 
    howto/formatting
    howto/opam-file-generation
+   cross-compilation
+   foreign-code
+   documentation
+   sites
+   instrumentation
+   jsoo
+   toplevel-integration
+   variants
+   tests
 
 .. toctree::
    :caption: Reference Guides
@@ -49,18 +58,9 @@ Welcome to Dune's Documentation!
    concepts/promotion
    concepts/package-spec
    coq
-   cross-compilation
-   foreign-code
    caching
    dune-libs
    rpc
-   documentation
-   sites
-   instrumentation
-   jsoo
-   toplevel-integration
-   variants
-   tests
 
 .. toctree::
    :caption: Explanations