Add streaming analytics doc page

This will probably become a blogpost (though not until the work is actually in to the various `*toolz` projects.)

@eigenhombre The join PR benefited significantly from your comments.  Your perspective would be appreciated here.
Although simple, I expect this example will still require a non-negligible number of readers to think for themselves to figure out what is going on.  `reduce` and `reduceby` can be difficult to grasp if one is not familiar with them and their API usage.

So, I would add a short sentence that explains what the above example does, such as:


The even numbers are added together (`2 + 4 = 6`) into group `True`, and the odd numbers are added together (`1 + 3 = 4`) into group `False`.