doc: build and install doxygen api docs

In order to support out of tree builds and avoid hardcoding version
number, generate `doc/config.dox` from configure.
doc: quiet doxygen warnings

remove some obsolete tags for XML output (which we don't currently
generate in any case)
doc: postprocess notmuch.3

Remove excess italics from doxygen output. It seems to make no
sense (and is certainly ugly) to italicize the first argument to the
.RI macro.