Document use of the individual.parent pedigree structure

[hyanwong]

At the moment, much of the detail is described in https://tskit.dev/msprime/docs/stable/pedigrees.html#pedigree-encoding. I think some of this is general to tskit rather than msprime, and so a short description of pedigree encoding in tskit is warranted.

[jeromekelleher]

We should also link to sgkit's pedigree methods for a compatible way of doing more calculations. We would want a conversion function somewhere, but I'm not sure where that would live.

See this discussion: https://github.com/tskit-dev/tskit/discussions/2711#discussioncomment-5069895