Add sub-sections to main User Documentation section

The list of topics in the User Documentation section on the main index page is way too long and very difficult to use.  I added subsection headers and did a small amount of re-ordering.  This is my attempt to make this important landing page more useful.   For a preview see: 

http://hea-www.harvard.edu/~aldcroft/tmp/astropy-docs/html/index.html

I did the subsection headers as plain lines of bold font instead of using the next level of ReST header, because these would be too big (in my opinion).

Some of the subsection headers are a little informal.  I think that's OK but I won't mind if others disagree and want to change them.