Document where to actually find the scripts menu
because it took me much longer than it should have.

I have the lua-scripts repo cloned in $HOME/.config/darktable/lua

Since darktable a4003bc
(darktable-org/darktable@a4003bc)
(PR darktable-org/darktable#20848) removed the direct invocation of
the user luarc from lua/init.c, that responsibility moved to
script_manager.  But it was only called in the `if system_based` case,
so my luarc was never loaded.

This commit pulls the user luarc script loading up so that it runs in
both modes.

Document where to find script manager in UI

Load user luarc files for users that have their own lua-scripts