Link to afw Box docs from geom

While looking for the Box2I vs. Box2D description document for @Lauren MacArthur in lsst.geom, KT pointed out that the doc is still in afw: https://pipelines.lsst.io/modules/lsst.afw.image/indexing-conventions.html#floating-point-and-integer-bounding-boxes

That description includes a lot of discussion about the relationship between Box and Image, so it probably belongs in afw, but we should at least link it from lsst.geom.