docs: Add docs to point to S3 or GCS instead of MinIO

Created by: efritz

This adds a section to the old "external databases" instructions on how to use S3 or GCS instead of the MinIO container that's bundled with the deployment. This became too big so I split it into a sub directory and updated all inbound links.

The deployment environments have not yet been updated to use these environment variables, but will be after the open PRs to add the minio container are merged (https://github.com/sourcegraph/sourcegraph/pull/15062, https://github.com/sourcegraph/deploy-sourcegraph-docker/pull/169, and https://github.com/sourcegraph/deploy-sourcegraph/pull/1158).