Reference manual reorg

[Neil Jerram]

As keen CVS followers will know, I've begun reorganizing the reference
manual (in the unstable branch), with one aim being to separate the
pure API reference material from more general guidance and
introductory/didactic material.  As part of this reorg, the "Guile
Scheme Reference" part becomes the "Guile API Reference" and is
widened to cover the C API as well as Scheme.

If anyone thinks this is desperately, tragically wrong, please speak
up!

        Neil