Adding a banner to the new MMTEB leaderboard (#1908)

* Adding a banner to the new MMTEB leaderboard

* linting

* Update mteb/leaderboard/app.py

Co-authored-by: Isaac Chung <chungisaac1217@gmail.com>

* adding reference to mteb arena

---------

Co-authored-by: Isaac Chung <chungisaac1217@gmail.com>

mteb/leaderboard/app.py

@@ -234,6 +234,12 @@ def filter_models(
 """
 
 with gr.Blocks(fill_width=True, theme=gr.themes.Base(), head=head) as demo:
+    gr.Markdown("""
+    ## MMTEB: Massive Multilingual Text Embedding Benchmark
+
+    The MMTEB leaderboard compares text embedding models on 1000+ languages. Check out the [paper](https://openreview.net/pdf?id=zl3pfz4VCV) for details on datasets, languages and tasks. And you can contribute! 🤗 To add a model, please refer to the documentation in the [GitHub repository](https://github.com/embeddings-benchmark/mteb/blob/main/docs/adding_a_model.md). Also check out [MTEB Arena](https://huggingface.co/spaces/mteb/arena) ⚔️
+    """)
+
     with gr.Row():
         with gr.Column(scale=5):
             gr.Markdown(