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/clear-payload/simple/java.java

@@ -0,0 +1,13 @@
+package com.example.snippets_amalgamation;
+
+import static io.qdrant.client.PointIdFactory.id;
+
+import java.util.List;
+
+public class Snippet {
+        public static void run() throws Exception {
+                client
+                    .clearPayloadAsync("{collection_name}", List.of(id(0), id(3), id(100)), true, null, null)
+                    .get();
+        }
+}