In fact the forum new organization is not finished yet.

I would like to remove Support/Extensions (ie moving all its content either in Support/Troubleshooting or in PB/Engine or PB/TT).

Please note that Engine != core. Engine is about general coding in Piwigo. Today the main activity is plugin coding (and that's a good thing), but unfortunately the previous Extensions section was containing some user related topics and some coding related topics. They should not be mixed.