Add basic component/flow diagram.

Adds the overall architecture flow for the st2 platform.

Fixes: https://github.com/StackStorm/discussions/issues/13
Update overview.rst
Merge branch 'master' of https://github.com/StackStorm/st2 into architecture-doc
properly render architecture diagram (h/t @kami)

This commit adds a static asset directory under `docs/source/_static`. Used
to serve out the architecture diagram.
Merge branch 'architecture-doc' of https://github.com/StackStorm/st2 into architecture-doc

Conflicts:
	docs/source/overview.rst
Merge pull request #862 from StackStorm/architecture-doc

Add basic component/flow diagram.