doc/css: add scroll-margin-top to dt elements

doc/css: add scroll-margin-top to dt elements

add "scroll-margin-top: em3;" to custom.css so that the header bar
doesn't obscure the text of headwords in glossary.rst. Note that this
applies only to elements in the documentation that are rendered into
HTML with the dt (which stands for "description term" or "description
list") tag. Other modifications will be necessary in order to ensure
that the anchor points of non-dt elements are not obscured by the header
bar.

Signed-off-by: Zac Dover <zac.dover@gmail.com>