Add docs for OCI setup and move schema docs to right place

[achantavy]

Feature request template

Title: One line description

Description:

Describe your idea. Please be detailed. If a feature request, please describe the desired behavior, what scenario it enables, and how it would be used.

1. We don't have setup docs for OCI configuration. We should have setup docs for OCI configuration :)
2. OCI schema docs are here: https://github.com/lyft/cartography/blob/master/docs/schema/oci.md, but they should be here: https://github.com/lyft/cartography/tree/master/docs/root/modules