Move mechanisms to override stack docker images to profiles

There are currently two main ways to override docker images used by the compose provider of `elastic-package stack`:
* Use the `image_ref_overrides` setting in the `.elastic-package/config.yml` file.
* Use the `*_IMAGE_REF_OVERRIDE` environment variables.

Both mechanisms are undocumented, barely supported and tested, and are not local to profiles, while the rest of stack resources and settings are.

We should establish a single supported mechanism for this, that works more like a profile setting, as other settings we have been recently adding (see https://github.com/elastic/elastic-package/pull/1211). And we should document it.

Review the existing image_ref_overrides setting in .elastic-package/config.yml and the *_IMAGE_REF_OVERRIDE environment variables, then compare their behavior with the profile settings described in PR 1211. Define one profile-local supported mechanism for overriding compose-provider images, remove or deprecate the undocumented alternatives as appropriate, and document the resulting setting.

Written by the indexing model from the issue text.