Add Stepper guideline

[Cuervino]

Summary

• Add stories/stepper.guideline.mdx with usage rules, typical pattern (form → execution → summary), back navigation note, and illustrated step states
• Update storySort in .storybook/preview.tsx so the Guideline tab appears before stories in the sidebar for the Stepper component
• Add StateCompleted, StateInProgress and StateError stories to stepper.stories.tsx (hidden from sidebar via tags: ['!dev'], used only by the guideline Canvas blocks)
• Improve the SimpleStepper story layout: buttons anchored at the bottom with stable width, no layout shift when navigating between steps

[bert-e]

Hello cuervino,

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Status report is not available.

[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

• the author

• one peer

Peer approvals must include at least 1 approval from the following list:

• @ZiyangLinScality

• @hervedombya

• @JeanMarcMilletScality

• @damiengillesscality

• @MonPote

• @ChengYanJin

• @JBWatenbergScality