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/update-collection/trigger-indexing/java.java

@@ -0,0 +1,15 @@
+package com.example.snippets_amalgamation;
+
+import io.qdrant.client.grpc.Collections.OptimizersConfigDiff;
+import io.qdrant.client.grpc.Collections.UpdateCollection;
+
+public class Snippet {
+        public static void run() throws Exception {
+                client.updateCollectionAsync(
+                    UpdateCollection.newBuilder()
+                        .setCollectionName("{collection_name}")
+                        .setOptimizersConfig(
+                            OptimizersConfigDiff.getDefaultInstance())
+                        .build());
+        }
+}