Unifying stat method desc and linking to math summary

[gregorgorjanc]

I was working through the stats tutorial at https://tskit.dev/tutorials/analysing_tree_sequences.html (incoming PR) and thought that linking stats/math summaries (https://tskit.dev/tskit/docs/stable/stats.html#summary-functions) to method docs would be very useful.

I also followed a style from the diversity method doc (what is the stat, then links to sample_sets, windows, ...).

@petrelharp can you have a look at this PR?

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[petrelharp]

In general I like the rearrangments, but I don't think we should add "please see summary functions for a precise definition". Instead, we should make what's said there in the documentation precise enough, if it isn't already? I am pretty sure that what's there for diversity, for instance, is precise - did you find it ambiguous? The 'summary function' definition is nice and concrete once you understand it, but most readers won't be on board with this. Some other way of referring to the summary functions could be helpful but more like "if you want the computational details, see XXX"?

[gregorgorjanc]

@petrelharp I followed your suggestions in the last commit.

I thought adding the link to computational/mathematical definition would be useful to provide concrete/mathematical detail of what is calculated. This makes things concrete, but since it's only a link, it does not overwhelm a user (I agree that simple descriptive text should be preferred). Also, you have written these math details for some stats, so adding these links exposes/leverages that part of the doc (most users will only look at function docs and won't go over the whole stats doc to find math detail).

Happy to iterate.