[NFC] Export `jacobian_default_mode` (#1473)

Move and renames the internal `chose_qgt` from the qgt module to
`nk.jax.jacobian_default_mode`, and properly documents it.

It turns out it's useful to implement NTK and other objects, and GPSKet
uses it as well, so let's make it public API together with Jacobian..

This changes nothing, simply moves a function (adds a deprecation
warning for the internal function in case anybody uses this function)
and adds more extensive docstrings.