Clarify supported asset types for Elastic package - content package type (asset-only)

## Overview

While trying to create content packages (asset-only) via the steps in the [documentation](https://github.com/elastic/elastic-package/blob/main/docs/howto/package_types.md) there is no mention of what asset types are currently supported.

For example, only some Kibana assets are supported, and does not include machine learning `ml_module`.

`Elasticsearch` assets are not supported, but this is not mentioned.

It would be beneficial for package creators to make the support explicit.