docs: Improve documentation of `s3` source

[udf2457]

In the docs, s3 is mentioned as a permitted source.

However, unless I've missed it somewhere, the docs are silent on how to deal with s3 credentials ?

[prestist]

The documentation on S3 authentication and other detailed operational notes are found in Ignition's
operator documentation here: https://coreos.github.io/ignition/operator-notes/#aws-s3-access

The spec documentation in Butane (the config-*.md files) is auto-generated from Ignition's schema
definitions. Those docs focus more on field definitions and structure rather than runtime behavior.

Since Butane is a transpiler and doesn't interact with S3 itself (that happens at boot time by
Ignition), the credential information lives in Ignition's docs.

However, I can see how someone writing a Butane config with source: s3://... would naturally wonder how authentication works. We could potentially add a note or link in the generated docs pointing to Ignition's operator notes for
runtime details, wdyt?

[udf2457]

I think that's a fair compromise, sort of a little bridge between the silos.