No-one is openly developing cores for the Pocket yet. Analogue has not opened anything up and trying to hack them is going to be an insurmountable challenge for an FPGA newbie, let alone people with experience.
The better place to start would be MiSTerFPGA. Fully open source and with a number of tutorials
to get you started. You can edit existing cores and maybe contribute a new feature here and there to get your feet wet. Downside of course is you need new hardware, but this is far and away the more developer-friendly platform and likely always will be. If and when Analogue opens things up 95% of your experience should immediately transfer to the Pocket since both platforms are based on variants of the Cyclone V.