Docs: Add streamlined a11y setup docs

[shilman]

We should be pushing users onto storybook add everywhere. This should:

1. Automatically install the correct version based on the version of Storybook they are using
2. Add it to both package.json and main.js today, and in the future add it to preview.ts
3. Run an addon-speciific postinstall script if it exists (e.g. @storybook/addon-test configures Vitest)

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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>

Greptile Summary

This PR streamlines the installation process for the Storybook accessibility addon (@storybook/addon-a11y) by introducing the 'storybook add' command, simplifying setup and ensuring version compatibility.

• Updated docs/_snippets/storybook-a11y-add.md with new 'storybook add' command for npm, pnpm, and yarn
• Removed docs/_snippets/storybook-a11y-register.md, eliminating manual addon registration
• Modified docs/writing-tests/accessibility-testing.mdx to reflect simplified addon setup process
• Improved documentation flow and removed redundant information in accessibility testing guide

[greptile-apps]

LGTM

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[jonniebigodes]

All good on my end. I've polished it up a bit and will merge this once the checklist clears. Thank you for taking the time to update the documentation and streamline the instructions. Appreciate it 🙏