Move s3 concepts.rst into examples directory

There was already a test for this, but we run our tests
against a git checkout of the repo.  However, docs/ is
not part of the sdist which explains why it could pass our
tests but fail to show up when people pip install'd the cli.

Fixes #280.

I did at least verify that removing the `docs/` dir caused
a test failure, and that with this change, the test passed.

Also manually verified that moving `docs/` then running
`aws s3 help` renders the expected content with this PR.