docs: sphinx: remove SPDX/copyright comments from HTML output

Use Jinja2 comments instead of HTML comments for the SPDX and copyright
lines. This prevents them from appearing in the HTML output, which was
never the intention; the HTML output has its own copyright line at the
bottom of the document.

Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20250830170808.3911151-1-vegard.nossum@oracle.com