Customer stories page: Side-bar topic chooser component

[machikoyasuda]

Separating this out from the main ticket #354

Component behavior

• User can click on any of the 6 links to be scrolled to the appropriate header area
• When the Impact stat section goes over the component, a slide gradient should appear

Research

• Double check a11y requirements any any ARIA labels, etc.

Styles

• Use the muted link style