Write out the TOC after all other documentation
Write out the TOC after all other documentation

The Makefile uses doc/spec/index.html as the target, so previously if
building any of the other documentation files failed, a subsequent run
of `make` would not retry, since doc/spec/index.html was up-to-date.

TelepathyIM/telepathy-spec,freedesktop-unofficial-mirror/telepathy__telepathy-spec