split up AWS index.md

The AWS index.md file is too big, too much written by developers as we go along, not for end users.

I propose splitting it into its own docs

• Intro
• S3A
• S3N
• S3 (branch-2 only, obviously)
• testing
• maybe in future: something on effective coding against object stores,
  though that could go toplevel, as it applies to all

I propose waiting for HADOOP-13560 to be in, as that changes the docs.