Make site building output deterministic

In order to do snapshot testing of the site output we want the output to
always be the same regardless of how the filesystem tree was created

omicron/ssg/site.py

@@ -185,7 +185,8 @@ class Site:
         self.other_outputs.append(pygments)
         self.add_by_path(pygments)
         content_path = self.site_path / "content"
-        for file in content_path.rglob("*"):
+        files = sorted(content_path.rglob("*"))
+        for file in files:
             if file.is_file() and is_content(file):
                 content = create_content(self, file)
                 if content.draft and not self.drafts: