Start populating the handbook with useful cross reference targets.

All cross reference labels start with name of the file that contains
them. A label for the top section level is simply the name of the
file (omitting the .sgml). Other references within the file append a
colon and onother name. For example, the label on the mailing list
section in the file eresources.sgml is eresources:mail. This gives
each file its own cross reference namespace.