Section headings without top border are not documented

[amyhupe]

What

2 new variants have just been added to the [Nature Section Heading] demo, showing headings without a top border.

We need to add guidance on when to use the different styles of section heading, and update the component description, which currently says: "It uses borders above and below the heading text to make it stand out from other headings on the page."

Why

To make sure the guidance is accurate and covers all relevant use cases.

Anything else

• Add link to guidance on using heading levels correctly, as we've noticed some issues with this on nature.com

[amyhupe]

Have made the guidance changes in this branch, but want to update the demo to show the subeading styles (with no top border) as h3s.

@sturobson is going to take a look.