Update doc links

Probably, I could have committed these directly, but it'd be nice to have someone take a second look and make sure I pointed some of the broken links to the right place.

I've also noticed that some links point to http://tutorial.obspy.org/path/to/something, but these are redirected to http://docs.obspy.org/tutorial and the /path/to/something is lost. I guess that's something that maybe @barsch might be able to fix up.