Make h3-h6 stand out

Currently, h3 is quite hard to visually recognize from normal paragraph text,
and h4 is only almost negligibly larger than the text, making them almost
impossible spot. As the headings should provide an easily visible separation
into sections, this adds a bit of weight on heading levels 3 to 6, making them
nicely visible and useful.

(Not that I'd want to use h6 for anything ( :D ), but h3 is still pretty okay
for subsectioning larger list-ish howtos.)
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