Make a couple organizational changes. Type no longer derives from Value

Make a couple organizational changes. Type no longer derives from Value
(hasn't for ages) so move it up one level in the table of contents.
Type needs to be understood before Value so move it before Value. Make
the descriptions of types stand out a little more.  Add references to the
doxygen for the Type class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33122 91177308-0d34-0410-b5e6-96231b3b80d8