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/overwrite-payload/by-point-id/java.java

@@ -0,0 +1,20 @@
+package com.example.snippets_amalgamation;
+
+import static io.qdrant.client.PointIdFactory.id;
+import static io.qdrant.client.ValueFactory.value;
+
+import java.util.List;
+
+public class Snippet {
+        public static void run() throws Exception {
+                client
+                    .overwritePayloadAsync(
+                        "{collection_name}",
+                        Map.of("property1", value("string"), "property2", value("string")),
+                        List.of(id(0), id(3), id(10)),
+                        true,
+                        null,
+                        null)
+                    .get();
+        }
+}