`config.Container` implements a multi-doc container which implements
both `Container` interface (encoding, validation, etc.), and `Conifg`
interface (accessing parts of the config).
Refactor `generate` and `bundle` packages to support multi-doc, and
provide backwards compatibility.
Implement a first (mostly example) machine config document for
SideroLink API URL.
Many places don't properly support multi-doc yet (e.g. config patches).
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
