Fix images in cirq docs

[dstrain115]

• Move images to an images directory
• Also use relative links to force docs to add them.

[vtomole]

Where does _static(https://github.com/quantumlib/Cirq/tree/master/docs/_static) fit into this?

[mpharrigan]

yeah maybe the images can be consolidated into _static but otherwise this looks like a good fix. Do the images show up if you run the notebooks locally?

[dstrain115]

Where does _static(https://github.com/quantumlib/Cirq/tree/master/docs/_static) fit into this?

Changed the images to _static and moved the links back to absolute.