Improve headline sizing flexibility in stepper component

[coderabbitai]

Context

This issue tracks feedback from PR #691 regarding headline sizing in the stepper component.

Problem

The current stepper component has hardcoded sizing for step titles, which can look problematic in certain scenarios:

• H3 headers used for steps appear too large
• Typically, content would have an H2 before the stepper, with H3 headers defining the steps
• Using smaller headers (e.g., H4) to reduce size results in MyST warnings: WARNING: Non-consecutive header level increase; H1 to H4 [myst.header]

Potential Solution

Consider making step title sizes configurable, potentially by adding a parameter to the stepper directive to allow users to control the visual size independently of the actual heading level.

References

• Original discussion: New stepper implementation using headers as steps #691 (comment)
• PR New stepper implementation using headers as steps #691: New stepper implementation using headers as steps #691
• Reported by: @amotl
• Discussed by: @bmunkholm

cc: @bmunkholm