Include prebuilt man pages again

Historically, Sphinx was required when building "standalone" builds,
else they would have no man pages.

This means that commit 0709e4be8b (Use standalone code paths by
default, 2025-10-26) broke man pages for users who build from a
tarball where non-standalone builds would use prebuilt docs.

Add a hack to use prebuilt docs again.

In future, we'll remove prebuilt docs, see #12052.

doc_src/conf.py

@@ -97,7 +97,12 @@ def setup(app):
     app.add_directive("synopsis", FishSynopsisDirective)
 
     app.add_config_value("issue_url", default=None, rebuild="html")
-    app.add_config_value("fish_help_sections_output", "", "man", str)
+    app.add_config_value(
+        "fish_help_sections_output",
+        default=os.environ.get("FISH_SPHINX_HELP_SECTIONS_OUTPUT", ""),
+        rebuild="man",
+        types=str,
+    )
     app.add_role("issue", issue_role)
 
     app.connect("builder-inited", remove_fish_indent_lexer)