[DOCS][PYTHON] Update embeddings API docs & Example (#516)

This PR adds an overview of embeddings docs:
- 2 ways to vectorize your data using lancedb - explicit & implicit
- explicit - manually vectorize your data using `wit_embedding` function
- Implicit - automatically vectorize your data as it comes by ingesting
your embedding function details as table metadata
- Multi-modal example w/ disappearing embedding function

python/lancedb/embeddings/__init__.py

@@ -21,5 +21,6 @@ from .functions import (
     OpenClipEmbeddings,
     SentenceTransformerEmbeddings,
     TextEmbeddingFunction,
+    register,
 )
 from .utils import with_embeddings