Addon-docs: Add legacy transitional support for MDX1

[shilman]

Issue: #20145

What I did

See attached migration notes:

If you get stuck with the MDX2 upgrade, we also provide opt-in legacy MDX1 support. This is intended as a temporary solution while you upgrade your Storybook; MDX1 will be discontinued in Storybook 8.0. The MDX1 library is no longer maintained and installing it results in npm audit security warnings.

Corresponding mdx1-csf PR here: https://github.com/storybookjs/mdx1-csf/pull/19/files

How to test

1. Run a Webpack sandbox for template, e.g. yarn task --task sandbox --start-from auto --template cra/default-ts
2. Rename Introduction.mdx to Introduction.stories.mdx
3. Edit the file to contain an MDX1-only pattern, such as:

<style>{`
  .subheading {
    --mediumdark: '#999999';
  }
`}</style>

4. Observe Storybook fails to start
5. Follow the install instructions in the attached MIGRATION.md
6. Observe Storybook starts successfully and you can navigate to Examples/Introduction story

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"]

[tmeasday]

Ok, these changes seem sensible.

So there's no way to transition partially from MDX1 -> 2? That seems OK if so.

[shilman]

@tmeasday unfortunately it's all or nothing for now. possible to enable piecemeal migration, but will require some API changes and i don't think it's worth the effort. this is really a stopgap solution and users should bite the bullet and do a full upgrade when they're ready.