1. Added an architecture section.
2. Added a configuration / execution sub-section to developer guide.

Minor tweaks and a bunch of missing fixes from `kafka-site` repo.

Author: Guozhang Wang <wangguoz@gmail.com>

Reviewers: Matthias J. Sax <matthias@confluent.io>, Derrick Or <derrickor@gmail.com>, Ewen Cheslack-Postava <ewen@confluent.io>

Closes #2488 from guozhangwang/KMinor-streams-docs-second-pass