add $ACCOUNT_ID environment variable for codecommit; minor instruction improvements

[fsiler]

couple tiny suggestions:

• jq could be installed by a cloud init script as part of the stack initialization
• a lot of these PNGs are unnecessarily large. I would crush them for you, but it would just make the repo even bigger. Recommend optipng -o7 before they even get committed; would be glad to write a commit hook if interested.