Ship rapi.rst/rapi.html in the dist archive

Patch 4352bf6d changed RAPI docs to RST from the previous SGML format
(the last file to be converted to RST).

However, it didn't add rapi.rst to the docrst Makefile variable, and as
such rapi.html was neither build nor shipped in the archive. We fix this
and we also distribute doc/rapi-resources.gen since otherwise “make
distcheck” has issues (since this file is built and thus lives in _built
whereas rapi.rst is shipped and lives in topsrcdir).

Note: yes, this means that no 2.0 release until now had a rapi.html file
(in the .tar.gz archive).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>