Bug 511589 - Doc bundles should have indexes generated at build time

In o.e.dltk.sh.doc:
* Add index to toc extension point
* Use tycho-eclipserun-plugin to generate the index at build time

Change-Id: I61c5d68355ee32457b390c62f3c1b13e08aebf4a
Signed-off-by: Mat Booth <mat.booth@redhat.com>