New tutorial: tskit for phylogeneticists / phylogenetics

[hyanwong]

As part of the website design, we want material to help people coming from the phylogenetic world, who might not realise that tskit can be used as a fast, scalable phylogenetic toolkit. It's not just tutorial material: there will presumably be some blurb etc on the website, but a tutorial is a good resource to have for this. There's a slight start in the "numba" stuff: #63.

@szhan might be able to contribute thoughts below, since he's coming from the phylogenetics world.

[hyanwong]

This would be useful as part of the website redesign. See https://docs.google.com/document/d/1GI1DsrOGdMKdigzsLiy2enTu7ToFN1v8cJ57btHWExY/edit#heading=h.fxh7sp5dc1cw for a suggestion