Docs: Document deprecation of storyshots

[kylegach]

Closes #22263

What I did

• Document deprecation of storyshots
• Add equivalent test-runner instructions

How to test

1. Follow the steps in the contributing instructions for this branch, storyshots-docs-migration
2. Review updates to:
   • Snapshot testing
   • Visual testing
   • FAQ

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>

[kylegach]

@yannbf — Should this be "index.json. mode"? The README uses that (and --stories-json isn't on the page), but the test runner docs page apparently hasn't been updated.

If your answer is "yes" to the above, I'll update the test runner docs, too (unless you tell me not to).

[kylegach]

@yannbf ?

[yannbf]

Hey! Index-json is preferred, as is the new way forward. stories-json is still available and works for Storybook 6 (which does not have an index.json file), but will likely be removed in the future

[jonniebigodes]

This is great work @kylegach ! Appreciate you taking the time to put together this pull request and provide some guidance to our readers for storyshots deprecation.

I left some minor items for you to look into, but on my end, this is good to go.