Update JJava configuration documentation

[stariy95]

See discussion here: https://github.com/dflib/jjava/issues/44#issuecomment-2417532573

In short: after #35 is done, we need to review configuration documentation. Also it may worth mentioning the possibility (and limitations) of manual editing the kernel.json file.

[stariy95]

As a note, here's an official Jupyter doc about kernel.json format: https://jupyter-client.readthedocs.io/en/stable/kernels.html#kernel-specs

[andrus]

done, the new docs are published - https://dflib.org/jjava/docs/1.x/#_configuring