Add doc directory to support distribution of CQL.html

[bschoening]

As part of CASSANDRA-17684, our proposed solution (in progress) will be to create a new build distribution target of 'doc', which will contain CQL.html and an empty init.py. This will allow cqlsh.py to locate and read the html file as a Python resource (>= 3.7) when cqlsh is pip installed.

See also Setup Tools Data File Support