docs(aws): updates per single-node/multi-node scenarios

[vdice]

Some AWS-related docs updates/re-org following https://github.com/fermyon/installer/pull/100 cc @FrankYang0529

• Re-introduce top-level AWS readme
• Remove some duplication from the single/multi-node readmes
• minor updates

[vdice]

@michelleN I believe I've addressed all comments -- let me know if there are any others to look at. Thank you!