Doesn't really affect the pages on the web, but it's easier to
preview them in the filesystem when linked to "../main.css" rather
than "/main.css".
The common portion of the <head> section is now in a separate
"include" file.
Added some indentation to masthead/topnav/sidenav so it looks nicer
in the merged source listing.
Fairly straightfoward in Python.
Also, fixed the sidenav references, which should be using the file
in the sgtutorial subdirectory.
Also, tweaked index.html a bit.
