Document how to work with a custom APN

[beriberikix]

@robthepyro on Discord struggled through an API change on the Nordic side while setting up a custom APN. We should document how to do that and provide feedback to Nordic as well.

tl;dr

# in prj.conf
CONFIG_PDN=y
CONFIG_PDN_SYS_INIT=y
CONFIG_PDN_DEFAULTS_OVERRIDE=y
CONFIG_PDN_DEFAULT_APN="flickswitch"

via https://discord.com/channels/819247807662915605/859829772992774154/906300879454482462