Autodocs: Hide Stories block when only primary story exists

[JReinhold]

Closes #20969

What I did

This PR adds a special case to the default DocsPage block for single story autodocs:

• Hide the "Stories" section below, to not show the primary story twice
• Move the primary story's description from the now hidden stories section to between the component description and the canvas.

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"]

[shilman]

LGTM!

[MichaelArestad]

@JReinhold Is there somewhere I can see an example of this in one of the test Storybooks?

[JReinhold]

After discussing this with @MichaelArestad we decided that the primary story's description should be featured below the component description in the case of single stories. See screenshots below, and Chromatic UI Review: https://www.chromatic.com/pullrequest?appId=635781f3500dd2c49e189caf&number=21178&view=changes

The only downside here is the fact that we now loose the narrative that "The DocsPage is super simple, you can copy it from here" because now it contains actual logic: https://github.com/storybookjs/storybook/pull/21178/files#diff-c940951c2adc4415d2b5dd98d12260a366f44fcd5ae7d4a3e92744ef780df9e8R11-R27
Any thoughts about that @shilman?

Single story

Multiple stories

[shilman]

I'm ok with this too, tho it lacks the simplicity of before. Please update the PR description before merging 🙏