Update tutorials/metadata.md

Partitioned metadata files into 100k rows (down from 1m). Metadata files also include a new column, source. Updated documentation to include source definition.

Adding small lines for embeddings.

Updating the examples to exclude s3 file keys, since those have now been removed.

Updating README.

Renaming dataset to pd12m.

Fixing broken link.

Updating README and adding tutorials.