sys_log: adds system log documentation page

Added sphynx comments required to link C code comments to rst
documentation, additionally a new rest page with the list of
current system log Kconfig options and an example.

Change-Id: I9d1370b5f0a2fbd858de83befb99f0f4c7024a13
Signed-off-by: Genaro Saucedo Tejada <genaro.saucedo.tejada@intel.com>
JIRA: ZEP-153