Docs: Add indexers API reference

[kylegach]

What I did

• Add API references for experimental indexers

Checklist for Contributors

Testing

Manual testing

1. Follow the steps in the contributing instructions for this branch, indexer-api-ref
   • Note that you'll need to ensure your frontpage:main is up-to-date, to get the necessary style updates
2. Review the new indexer page

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for 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:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 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>

[JReinhold]

@kylegach @jonniebigodes I've made some changes here to get it closer, but it's not quite ready yet:

1. converted the title snippet to a proper snippet
2. attempted to simplify the JSON stories example, and include an example JSON file as well as a psuedo Vite plugin
3. Changed the combos example to instead refer to the Dynamic stories POC, as it's an actual implementation of something that shows off a similar concept, so I think it's a stronger starting point for people. If you disagree let me know. (we should update the code with the new API when it's been released though)
4. added references to @storybook/addon-svelte-csf and storybook-vue-addon for the last example for custom language support. I think this is a very advanced use case that we can't document very well in our docs, so I hope referring to actual implementations is more valuable.

Let me know if you think we should do something else. @cdedreuille still has the ball on the diagram.

[jonniebigodes]

This is amazing, just some minor tweaks that need addressing. @kylegach let's sync up on this next week.

[kylegach]

@JReinhold — Please confirm the accuracy of these diagrams, as well as the more detailed text representation appearing under them.

[kylegach]

Also, can https://github.com/storybookjs/storybook/blob/next/docs/configure/sidebar-and-urls.md?plain=1#L164-L203 be replaced with a brief paragraph explaining that a custom indexer should be used, and link to this reference?

[JReinhold]

Looks good to me @kylegach ! and yes, I think it makes sense to change that too.