Skipnav should be a button

[flacoman91]

Describe the bug
Skip nav looks like a link in documentation site.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://cfpb.github.io/design-system/
2. Press Tab until the skip nav link appears
3. See error

Expected behavior
Skip nav should be a button

@sonnakim for awareness

Also, can this be a reusable component in the design system?

[flacoman91]

this will surface the component in DSR:

cfpb/design-system-react#473