docs: CSS: make cross-reference links more evident

docs: CSS: make cross-reference links more evident

The Sphinx Alabaster theme uses border-bottom to mark reference links; the
result does not render correctly (the underline is missing) in some browser
configurations.  Switch to using the standard text-underline property, and
use text-underline-offset to place that underline below any underscores in
the underlined text.

Suggested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>