Create docs for storage configuration

[pedro-psb]

Recently an RPM user reported that he couldnt find info about configuring S3.

There is a link in RPM docs that previously pointed to a storage page on pulpcore legacy docs, but that was not moved on docs migration. The reason is that we though it belonged here (more close to installation), but we failed to create an issue for this at the time.

Problem

Lack of information on available storages and on how to configure them (using pulp oci-images installation methods).

Proposal

Create a page in admin/guides/configure-storages.md based on ex-pulpcore storages page.