feat(docs): add multi-step form stepper prototypes

[gganch]

Adds a /tests/multi-step-form page to the docs site with six horizontal stepper prototypes for a multi-step form pattern. Each has numbered steps across the top, a blank LayerCard body, Back/Continue buttons, and a checkmark on completed steps.

1. Numbered circles with connectors
2. Segmented progress bar
3. Pill breadcrumbs
4. Kumo Meter with step markers
5. Tabs in the card header
6. Kumo Tabs (underline variant) inside a LayerCard, where you can click back to earlier completed steps — the option closest to existing Kumo patterns

Docs-only prototype: no changes to @cloudflare/kumo, so no changeset.

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• Reviews
  • bonk has reviewed the change
  • automated review not possible because: draft prototype for design feedback; bonk review will be requested before marking ready
• Tests
  • Tests included/updated
  • Automated tests not possible - manual testing has been completed as follows: ran lint and typecheck for kumo-docs-astro (0 errors), loaded /tests/multi-step-form in the dev server and clicked through each option
  • Additional testing not necessary because:

[gganch]

[gganch]