Fix stepper heading levels from snippets

[reakaleek]

Closes #2434

Problem

Stepper steps in included snippets always appeared as h2 in the table of contents, regardless of the parent document's heading structure.

Solution

When processing included snippets, we adjust stepper step heading levels based on the preceding heading in the parent document. Stepper steps that defaulted to h2 are adjusted to be one level deeper than the preceding heading.

Example:

## Main Section (h2)
:::{include} _snippets/stepper.md
:::

Stepper steps now correctly become h3 instead of h2.

Changes

• Added IsStepperStep flag to distinguish stepper steps from regular headings
• Adjusted stepper step levels when merging snippet ToC items
• Regular headings in snippets are preserved (not adjusted)