Fix HTML validation errors in docs - articles title section

article: render subtitle as <p> instead of <h3>

The subtitle is part of the title block, not a section heading.
Rendering it as <h3> right after <h1> broke W3C validation and
caused screen readers to announce it as a level-3 heading.

Move the visual styling from the <h3> tag to a .subtitle class
in documentation.scss to keep the appearance unchanged.

Reviewed by: carlavilla
Approved by: carlavilla
Differential Revision: https://reviews.freebsd.org/D60058
PR: 297509