[Halld] Hall D Web page - cosmetic changes

[Eugene Chudakov]

JLab is going to change soon the style and the substance of the top
WWW pages. The halls have been asked to provide a consistent look of
their pages (at least the top ones).

I modified the Hall D pages

https://www.jlab.org/Hall-D/

in order to imitate the new look. I also cleaned up some obsolete
stuff and added a few links. These pages are static in contrast to the
Wiki pages. Let me know if something does not work or is missing, or
if you would like to add some other links.

Eugene