[Ldraw-content] Added Parts Header Specification

[LDraw]

added "LDraw.org Parts Header Specification page":
http://www.ldraw.org/article/398

to "Specifications Main page":
http://www.ldraw.org/article/292

plus minor editing of the other titles + pages. please crosscheck. the 
following pages at the PT need an update:

http://www.ldraw.org/library/tracker/ref/faq/
http://www.ldraw.org/library/tracker/ref/catkeyfaq/

nontheless it is my believe that the specification section should be 
moved to the PT references. currently we have all the theoretical stuff 
at the main page, while the more usefully code snippets and examples at 
the PT. this is confusing! I suggest to hand over the entire 
specifications to the PT admins and link it from the front page.

w.