[SPARK-2419][Streaming][Docs] More updates to the streaming programming guide

- Improvements to the kinesis integration guide from @cfregly
- More information about unified input dstreams in main guide

Author: Tathagata Das <tathagata.das1565@gmail.com>
Author: Chris Fregly <chris@fregly.com>

Closes #2307 from tdas/streaming-doc-fix1 and squashes the following commits:

ec40b5d [Tathagata Das] Updated figure with kinesis
fdb9c5e [Tathagata Das] Fixed style issues with kinesis guide
036d219 [Chris Fregly] updated kinesis docs and added an arch diagram
24f622a [Tathagata Das] More modifications.

(cherry picked from commit baff7e936101635d9bd4245e45335878bafb75e0)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>