docs: document Backblaze B2 and S3-compatible storage for the s3:// filesystem

[goanpeca]

The s3:// file system supports Amazon S3 and any S3-compatible object store, but docs/overview.md did not document it. This adds an "S3 and S3-compatible object storage" section to the overview.

1. Credentials and region: documents the standard AWS environment variables (AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION) that the s3:// file system reads.

2. Non-AWS providers: documents pointing the client at a provider endpoint with S3_ENDPOINT (for example Backblaze B2, Cloudflare R2, MinIO).

3. Usage: notes that the same s3:// paths work for reading and writing through tf.data, tf.io.gfile, tf.train.Checkpoint, and tf.saved_model.

The variables and path-style addressing match the current tensorflow_io/core/filesystems/s3/s3_filesystem.cc implementation. Docs only; no code changes.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.