Add semantic headings to stepper component

[theletterf]

This PR updates the Stepper component so that:

• Step titles are rendered as an <Hn> heading, where n is the level of the preceding heading + 1.
• Heading thus appear in the table of content if they're of level 3.
• Step titles are not included in the ToC if they're nested inside another directive to avoid weird situations, such as competing headings in tabs using steppers and headings hidden inside a dropdown. Strange, but could happen.

Fixes #1464

Example of dynamic headings (h4 after an h3)

Example of H3 step titles appearing in ToC

[reakaleek]

nit: Wondering if this can be just .Descendants<StepBlock>()

[reakaleek]

Wondering if we also need to adapt the styling. Like adjust the size of the step number depending on the h-tag level. 🤷

[theletterf]

@reakaleek Good point. For now I would keep the number size the same.