Add the first versions of SDL Developer and User guides, which will
be published on the ORAN SC documentation page (under RIC project):
https://docs.o-ran-sc.org.
Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: Ic23568b44a1560ba59a5466aef2993f95d723894