Modules: a module with submodules is its folder's page

(std test) and (std stm) were pages next to folders of their submodules, and
the std menu lists folders first, so each showed up twice, far apart. The
module page is now the folder's index.sz, so the menu has one entry for it,
and its submodules are listed at the top of the page and in the menu below.

(std test prop), (std test gen) and (std test expect) get pages, and the
standard library chapter says what they have and links to them.

🤖 Generated with [ECA](https://eca.dev)

Co-Authored-By: eca-agent <git@eca.dev>

modules/std/test/index.sz

@@ -0,0 +1,11 @@
+(meta (title "(std test)")
+      (sort title))
+
+@strong{Submodules:}
+@itemlist{
+  @item{@link["prop.html"]{@code{(std test prop)}}: property tests with @code{for-all}.}
+  @item{@link["gen.html"]{@code{(std test gen)}}: the generators that @code{for-all} draws from, and @code{Arbitrary}.}
+  @item{@link["expect.html"]{@code{(std test expect)}}: tests that compare output with a text in the test.}
+}
+
+@generate-docs[#:module "std test"]