Vignette: Move "aims/target users" further up #106

Brieuc says: ... add a section near the top to spell out explicitly 'who this package is for' (which is more-or-less included in the 2nd paragraph of the introduction anyways) and then to point to the tskitr tutorial (https://tskit.dev/tutorials/tskitr.html) there (since that's currently a bit hidden in the next 'State of the tree sequence ecosystem' section).