Docs for `quantile_cont` do not describe how interpolation is calculated

[david-cortes]

In the docs for function quantile_cont: https://duckdb.org/docs/sql/functions/aggregates.html#quantile_contx-pos

It mentions that it returns an interpolated quantile, but doesn't describe the method that is used to interpolate, nor does it allow controlling it.

See for example the docs for R's quantile for examples of potential definitions for the interpolation: https://www.rdocumentation.org/packages/stats/versions/3.6.2/topics/quantile