Replace top-level About page by "features" page.

The About page contained placeholder text from Wikipedia.  When I tried
to rewrite it, I only ended up reiterating the same points as on the
features page.  Move the features page up to the top-level.

The introductory paragraph is different, and more or less lifted from
Mercury papers.  It says Prolog once instead of three times.

include/about.inc:
include/menubar.inc:
about/include/features.inc:
	As above.
Rename About "overview" page to "motivation".

about/include/motivation.inc:
about/motivation.php:
include/menubar.inc:
	As above.