Move reference documentation for "Variant" class

[hyanwong]

At the moment the Variant class is listed under the "simple container classes" heading, here: https://tskit.dev/tskit/docs/stable/python-api.html#the-variant-class but that doesn't seem like a sensible description any more. We could move it somewhere else, or change the name of the heading.

[benjeffery]

Here I propose creating a new section under "Analysis" which details accessing variant data - and slimming down the docstring of the Variant class.