devel/libsoup: add a DOCS option

Add DOCS, which just uses gtk_doc to do the work. The change
was suggested by Sean Champ in the linked PR, but I reworked
it substantially to be explicit about the files, and to use
the simplest tooling. Other changes suggested by the way
devel/libical and devel/json/glib do DOCS with gtk_doc.

PR: 265568
Reported by: Sean Champ