Update task category and add relevant tags for classification and retrieval

This PR updates the `task_categories` metadata from `fill-mask` to `feature-extraction`. This change better reflects the primary use cases of models trained on this dataset, which involve extracting features for downstream tasks like classification and retrieval, as highlighted in the paper abstract and GitHub README.

Additionally, I've added `text-retrieval` and `text-classification` to the `tags` to further improve discoverability for users interested in these specific applications.

README.md

@@ -1,11 +1,13 @@
 ---
 license: mit
 task_categories:
-- fill-mask
+- feature-extraction
 tags:
 - pretraining
 - encoder
 - multilingual
+- text-retrieval
+- text-classification
 ---
 
 # MMBERT Decay Phase Data