document the license dataset on docs.s.o

The license dataset is currently available from the annex (and documented there using a README file), but not documented/visibile on the main documentation page of swh-dataset. We should fix this by adding/moving the documentation there.

Migrated from T4551 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information