Heading docs are misleading / confusing

[BinaryMuse]

The docs for the Heading component say:

The Heading component will render an html h1-6 tag without any default styling.

However, this component always renders a h1 unless the user passes as="h2" etc, which is not made explicit in the docs. We should:

1. Change Heading to default to h2 so users don't accidentally use multiple h1s
2. Update the docs to be clear on how to choose a different heading level

[taylorcjohnson]

Hello, if you’re open to new contributors I would like to volunteer for this issue! This would be my first contribution to Primer, but the guidelines look straight-forward.

[emplums]

Done!