rtw89: add man page and hook driver up to the build

After two years, add the initial man page and hook Realteks rtw89
driver up to the build for more people to test given successful
reports.
devd.conf is already providing support based on the rtw<n+> regex.
The driver uses the LinuxKPI compat layer.

Firmware is provided by ports: net/wifi-firmware-rtw89-kmod or
one of the flavours. People are advised to used fwget(8) to
automatically install the correct firmware for their chipset.

Please note that for the moment the driver requires a tunable to
be set in loader.conf: compat.linuxkpi.skb.mem_limit=1

Many thanks to everyone who in the last two years helped testing,
debugged, submitted patches to get the driver to this stage where
initial functionality seems working.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days