git ssb

Reduce the main line-height from 2rem to 1.5rem

This makes the body text much more readable,
especially in narrow viewports.

It also makes the vertical rhythm much less rigid.
There are now multiples of 1rem and 1.5rem,
but nothing that's just 0.5rem on its own
(except the gap between the logo and its hover/focus border).

Other elements are unchanged:
the logo is still 4rem;
the page title's line-height is still 2rem;
pre line-height is still 1rem.

Line-height for headings in articles is still 2rem.
(This required an extra style declaration.)
First-level heading text is 1.7rem,
and the extra space isn't harmful for lower-level headings.