Simplify quickstart by providing a pubkey and signed/unsigned images

[znewman01]

Fixes #1 (well, except the fancy GIF).

Contains miscellaneous fixes/cleanups/docs, but the key changes:

• allow providing the key to use as either an ARN or a full PEM-encoded key.
• provide a hardcoded key that we've used to sign/not sign two images in our public ECR registry
• use those images/that key by default in the Makefile

This means that our quickstart is now (paraphrased): make sam_deploy && make tf_init && make_tf_apply, plus some instructions for checking that it worked.

The previous content and new instructions for configuration are now in the README below. Feel free to give copyediting/nitpicky feedback here!

[znewman01]

@strongjz LMK if this looks good to you