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/insert-points/with-custom-shard/java.java

@@ -0,0 +1,33 @@
+package com.example.snippets_amalgamation;
+
+import static io.qdrant.client.PointIdFactory.id;
+import static io.qdrant.client.ShardKeySelectorFactory.shardKeySelector;
+import static io.qdrant.client.VectorsFactory.vectors;
+
+import io.qdrant.client.QdrantClient;
+import io.qdrant.client.QdrantGrpcClient;
+import io.qdrant.client.grpc.Points.PointStruct;
+import io.qdrant.client.grpc.Points.UpsertPoints;
+import java.util.List;
+
+public class Snippet {
+        public static void run() throws Exception {
+                QdrantClient client =
+                    new QdrantClient(QdrantGrpcClient.newBuilder("localhost", 6334, false).build());
+
+                client
+                    .upsertAsync(
+                        UpsertPoints.newBuilder()
+                            .setCollectionName("{collection_name}")
+                            .addAllPoints(
+                                List.of(
+                                    PointStruct.newBuilder()
+                                        .setId(id(111))
+                                        .setVectors(vectors(0.1f, 0.2f, 0.3f))
+                                        .build()))
+                            .setShardKeySelector(shardKeySelector("user_1"))
+                            .build()
+                    )
+                    .get();
+        }
+}