Fix invalid HTML in docs hero (button nested in link)

A <button> is interactive content and isn't allowed as a descendant of
<a> per the HTML spec. Replace the link-wrapped buttons in the docs hero
with styled anchors so the markup validates.

Closes #62

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mblode