Improve headline sizing flexibility in stepper component

## 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: https://github.com/crate/crate-docs-theme/pull/691#discussion_r2714674886
- PR #691: https://github.com/crate/crate-docs-theme/pull/691
- Reported by: @amotl
- Discussed by: @bmunkholm

cc: @bmunkholm