- the first HTML comment in those files is extracted as page
  title when serving them
- lib/godoc.html is top-level template for all pages served
- experimented a bit with package documentation layout
  (feedback welcome)
- various related cleanups

TODO:
- The / page (doc/root.html) content repeats links that are
  in the navigation bar. It needs to be cleaned up.

R=rsc
DELTA=826  (86 added, 692 deleted, 48 changed)
OCL=35230
CL=35245