Create an "IBD" documentation page in the "Analysis" part

[hyanwong]

Once IBD methods and classes are documented, we decided to create a page called "Identity by descent" under the part of the tskit docs labelled "Analysis". Akin to the (new) "Topological Analysis" section, this should give some quick demos of how to use the IBD classes (see #1951)

It can be a bit of a stub for now, I guess

[jeromekelleher]

I can do this as part of #1902, unless you've done it already?

[hyanwong]

I haven't done anything. Just opened up this issue because we had forgotten to document that we were going to do it. Do please add it as part of #1902