Indexers: Mark indexers as experimental

[JReinhold]

Works on #23457

Telescopes on top of #23660

What I did

Changed the preset property from indexers to experimental_indexers. They are still indexers internally everywhere, only the preset.apply calls and StorybookConfig type changed so if users were to add their own indexers, they'd need to add them as experimental_indexers in main.ts.

I didn't change all the new types that the indexers use, that didn't seem necessary. I also didn't add a console warning given that we're adding default indexers which would trigger this warning every time.

I also reverted my updates to the code snippets, as I think they should get updated when we write the new docs at the end of this project, and not before that.

How to test

1. Create a sandbox.
2. Add an experimental indexer in the main.ts file:

...
  experimental_indexers: (existing) => {
    console.log('👋 from experimentals.');
    return existing
  }

3. Start Storybook and see the log in your terminal.

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

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts
• Make sure this PR contains one of the labels below.

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

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

[jonniebigodes]

Thanks for this @JReinhold , and agree with the docs changes. We can revisit them later on.