Automated snippet conversion by scripts

These changes are purely mechanical to differentiate them from manual
fixes/adjustments made in the next commit.
This results in broken code as some snippets contain errors.

Made in four steps:
1. Run ./migrate-snippet.py that converts `.md` files to code files
   and perhaps adds missing lines under `// @hide` comments.
2. Sort Java imports.
3. Remove old `.md` files.
4. Run ./generate.md to produce `*/generated/*.md` files.

qdrant-landing/content/documentation/headless/snippets/retrieve-points/simple/python.py

@@ -0,0 +1,8 @@
+from qdrant_client import QdrantClient  # @hide
+
+client = QdrantClient(url="http://localhost:6333")  # @hide
+
+client.retrieve(
+    collection_name="{collection_name}",
+    ids=[0, 3, 100],
+)