Make skip link look less "bare bones"

Additional markup for the skip link (to ensure it has the same effective
width as the main content, and to provide an outline around just the
link text) plus some basic styles...make it look a bit more in keeping
with the overall look and feel of the docs.
Expand/clarify advice about skip links
Merge pull request #15363 from patrickhlauke/docs-nicer-skip-link

Make skip link look less "bare bones"