docs(alertbanner): migrate docs to storybook

[jawinn]

Description

Create docs-only stories for alert banner in Storybook to represent what was on the docs site.

Modifies the alert banner's action button control to allow the text of the button to be editable.

Adds unrepresented test cases and examples for options with just an action button (no close button), and text only (no action button or close button). The design team had confirmed these as available options during the S2 migration.

Note: these updated docs were based on the ones from from the WIP S2 migration; I decided against using the custom MDX file and just prefixing the three variant stories with "Variant: " to group them together, since the only unique thing the custom MDX was providing was another heading above the three semantic variants. And because the current preference is to use the autodocs template unless it's really necessary to create a custom MDX.

How and where has this been tested?

Please tag yourself on the tests you've marked complete to confirm the tests have been run by someone other than the author.

Validation steps

• Docs from docs site are represented in the Storybook "Docs"
• All existing sets of options displayed on the chromatic template are still covered in the updated template
• New chromatic template covers options for both buttons, just close button, just action button, and text only

Regression testing

Validate:

1. The documentation pages for at least two other components are still loading, including:

• The pages render correctly, are accessible, and are responsive.

2. If components have been modified, VRTs have been run on this branch:

• VRTs have been run and looked at.
• Any VRT changes have been accepted (by reviewer and/or PR author), or there are no changes.

To-do list

• I have read the contribution guidelines.
• I have updated relevant storybook stories and templates.
• If my change impacts other components, I have tested to make sure they don't break.
• If my change impacts documentation, I have updated the documentation accordingly.
• ✨ This pull request is ready to merge. ✨

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Latest commit: 0c76468

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Summary

Total size: 4.11 MB^*

🎉 No changes detected in any packages

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[rise-erpelding]

This looks great! Test coverage looks good, info from Docs Site is included, and the variants are all displayed in a really nice way. Well done!

I added one tiny grammatical suggestion but don't expect this would need a re-review afterward, so I'm approving!

[castastrophe]

Great addition to the docs page, thanks for linking the design docs. Let's make a note to publish the site after this merges. Thanks for making these updates and expanding the test coverage.