Add ibd tutorial stub

[jeromekelleher]

We'll want to have a tutorial for IBD related stuff at some point, so useful to be able to refer to it from tskit's docs. Add a stub here.

I guess we'd add it in the STATISTICS AND ANALYSIS part @hyanwong?

[hyanwong]

I don't know if we want it to be part of the "Analysing tree sequences" tutorial, which I thought could cover both single site stats and multi-site stats. I don't know if we consider the IBD stuff to be a "stats" framework though? And anyway, there might be too much to put in a single "Analysing tree sequences" tutorial: Perhaps we should break it down into "single site statistics", "multi-site statistics", etc?

[jeromekelleher]

I think we can make its own tutorial, "Identity by descent" - there'll be enough in there, eventually.