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/delete-collection/simple/rust.rs

@@ -0,0 +1,7 @@
+
+
+pub async fn main() -> anyhow::Result<()> {
+    client.delete_collection("{collection_name}").await?;
+
+    Ok(())
+}