doc: fix substitution with mdbook 0.4

Recently, in 54df89f601, support for mdbook 0.5
was introduced, including some logic to handle the `sections` -> `items`
rename. However, compatibility with 0.4's `sections` was only kept on the read
path, while writing 0.5's `items` unconditionally, which ends up in the bit
bucket on 0.4, effectively disabling substitution fully and leaving the include
directives in the final documentation. Restore writing into the `sections` when
they were there so that substitution works again.

Change-Id: Idd4d7653012660f3f7fc27f81f29b82d6a6a6964

doc/manual/substitute.py

@@ -71,7 +71,11 @@ def do_include(content: str, relative_md_path: Path, source_root: Path, search_p
 def recursive_replace(data, book_root, search_path):
     match data:
         # XXX FUTURE: drop sections once mdBook is at 0.5.0 or above in nixpkgs
-        case {'sections': items} | {'items': items}:
+        case {'sections': sections}:
+            return data | dict(
+                sections = [recursive_replace(section, book_root, search_path) for section in sections],
+            )
+        case {'items': items}:
             return data | dict(
                 items = [recursive_replace(item, book_root, search_path) for item in items],
             )