doc: make headings more visually identifiable

Before this change, headings in documentation pages were using the same
font family as the rest of the content, which made them blend in too
much.

This introduces a humanist font stack for the headings as well as some
minor tweaks to make the hierarchy of the document clearer through
better differentiation of the different levels (i.e. not just relying on
font size).

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
1 file changed