Documentation: Remove subcomponents feature

[JReinhold]

What I did

This PR removes any mentioning of subcomponents from the docs, as it is deprecated in 7.0.

The biggest docs page to focus on is https://storybook.js.org/docs/7.0/react/writing-stories/stories-for-multiple-components which starts by mentioning subcomponents and then lists a few more powerful alternatives. I've tried to lightly re-write this To only mention the alternatives, but the structure or wording might not be perfect.

I hope that @jonniebigodes can move this PR forward and bring it over the finish line.

How to test

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• If this PR should be tested against many or all sandboxes,
  make sure to add the ci:merged or ci:daily GH label to it.
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "documentation", "maintenance", "dependencies", "other"]

Deprecation

For future reference, here's a quick summary on the reasoning behing the deprecation:

subcomponents is deprecated in 7.0. The API is very limiting and hard to improve on. It is somewhat of a half-baked feature, that we want to rethink and do better.

Subcomponents has been removed from the default DocsPage in 7.0, but since it's still available in the now deprecated doc block, they can be re-added by defining your own DocsPage globally.

We don't have a clear path forward from this yet, we want to collect feedback from the community about what they want from their subcomponent documentation and how they want to use it, both for docs and for development.